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[Javascript]Fill a textbox without ID and a name that changes on page reload using a bookmark

So I am trying to make a bookmark in chrome that would input a Javascript code that would fill up a textbox on my router home page. The goal is saving me the hassle of either remembering that silly password or having to open my textfile containing the said password.
I know, I know, ... I am lazy. (but not for learning some Javascript in the process)
The thing is the textbox doesn't have an ID and its name changes on reload so I cannot know its name in advance.
I have looked at a few pages on here that kind of guide me in the right direction but I can not make it work for the life of me as I have little to no experience in Javascript.
Here is what the html of the textbox looks like :
<input type="PASSWORD" style="WIDTH: 150px" name="password_random10digitnumber" value="" size="20" maxlength="64">
Here is what I ended up with as a link on my bookmark (the only thing I tried that doesn't give me an error).
javascript:document.getElementsByTagName("input")[0].value='myrouterpwd'
Currently, when I press my bookmark, it refreshes the page and shows a blank page with "myrouterpwd" on it.
(There is two other input html blocks on the page which are :
<input type="HIDDEN" name="md5_pass" value="">
<input type="HIDDEN" name="auth_key" value="numbersIamnotsureIshouldshowtheworld">
)
Thank you to anybody taking the time to answer!

I haven't worked much with bookmarks and am assuming that because it's a bookmark, you are navigating away from the page that you actually want to work with. So, I'm not sure that triggering this that way is a viable solution. Of course, you could just take advantage of the browser's ability to store passwords for you. ;)
But, to the main point of your question... Assuming that it's the only password field on the page, the use of element.querySelector() will find it:
// Get a reference to the right element based on its type attribute value
let passwordBox = document.querySelector("input[type='password']");
passwordBox.value = "MySecretPassword"; // Populate the field
console.log(passwordBox.value); // Confirmation
[type='password'] { background:yellow; } /* Just so you know which box is the password box */
<input type="hidden" name="doesntMatter">
<input name="someTextBox">
<input type="password" name="doesntMatter2">
<input name="someOtherTextbox">

PHP redirect with header

I'm not expert in PHP, and I'm trying to create public chatroom for my simple website.
I'm using SQL database to store messages.The file named chat_index.php getting all messages from database and show it to users. Also it has a simple form to send message with PHP GET method. The following is code for my form.
<form method="get" action="sendmessage.php">
<input name="msg" placeholder="Message" data-theme="a" type="text">
<button type="submit" class="ui-btn ui-corner-all">Send</button>
</form>
With above code I'm sending data to sendmessage.php file. In this file adding message to database and user redirect to chat_index.php with this code.
header("Location: chat_index.php");
exit();
After redirect page loading correctly on browser window. But URL end like this
...../sendmessage.php?msg=test_message
So if I reload the page message sending again and url getting correct like this
...../chat_index.php
How can I resolve this problem?
UPDATED
I tried with POST method. but not solved. browser showing content in chat_index.php and url ending with ../sendmessage.php

Why you are recieveing the url variables is because you are using GET. use POST instead and the variables will be gone.
example:
<form method="POST" action="sendmessage.php">

If you don't want to send data to the query string use POST instead of GET.
<form method="POST" action="sendmessage.php">

This should clearly be a POST form. Any form saving or changing anything on the server should be. Otherwise (using get) people could add messages by just following a link, which is not intended. The redirect after save is OK and good practice. Note that such redirects only work before any output is produced.
Cannot say much more by what you have posted. You should also check if your saving and redirect code is triggered correctly when message is posted.

This is what is happening, the form is being submitted from chat_index.php, so the action ends up being chat_index.php/sendmessage.php.
To solve this change the method to post as directed and change your action to /sendmessage.php
your form should look look like this
<form method="POST" action="/sendmessage.php">
....
</form
hope this help

If this is to be a simple forum site, then it would be much less complicated to not redirect at all. Just set your action attribute to "", and when someone submits the form, it will post the data back to the page. So do something like this:
<form method="post" action="">
<input name="msg" placeholder="Message" data-theme="a" type="text">
<input name="function" type="hidden" value="post_message">
<button type="submit" class="ui-btn ui-corner-all">Send</button>
</form>
Then put the php code that was in sendmessage.php into chat_index.php above the starting DOCTYPE and html tags, and delete sendmessage.php. Or alternatively, use include('sendmessage.php') in the same spot. Then when the page loads, check if $_POST['function'] == 'post_message', and if this condition is true, then execute the stuff you had in sendmessage.php. That will be much more compact, and the user will be redirected once instead of twice. Also, I don't know your file structure, but you might want to rename chat_index.php to just index.php to make it intuitive and so that people can't see inside your directory.

How do i make a link appear in a textbox, and let the button save it in a database

I'm about to create a google extension, where i simply wanna make something like a bookmark app. Just to learn.
I did now fall over my first issue, when i could not find no help for this, so i'll try here:
I right now have a simple form looking like this:
<div class="form">
URL: <input type="text" name="url"><br>
</div>
What i want it to do, is i want it to display the exact link where people opens my app! Picture
When that is done, and people clicks the save button, i want the link to be stored in a mysql database. How am i supposed to do that? Any answar will be taken with a smile.
UPDATE! I found out that i can't use php as file format for the extension. It have to be .html. I can include javascript, so if that's possible, i would like to know how.

I don't know how you retrieve your link, so I'll skip that part and just assign it to a variable...
<?php $url = 'http://www.stackoverflow.com'; ?>
<div class="form">
URL: <input type="text" name="url" value="<?php echo (isset($url))? $url:''; ?>"><br>
</div>
To save by just a onblur or onkeyup you can use AJAX.
If you want to click a submit button, then you just write an if condition to save it to your db like so:
if(isset($_REQUEST['url'])) {
// do insert into database here...
}

How to change iFrame results to JavaScript alert box?

I have access to a url runs a script to clear a users state.
Id like to produce a script that will run on a webpage, to do this.
The following script works but in Firefox its annoying for people to have to disable mixed content each time they come to the page:
<SCRIPT LANGUAGE="JavaScript">
function go(loc,loc2){
document.getElementById('userstate').src = loc +
document.getElementById('username').value + loc2;
}
</script>
<form onSubmit="go('http://mysiteurlomitted.com/userstate/?userId=','&app=cc'); return false;"/>
Username: <input type="text" id="username">
<input type="submit" value="Clear User State">
</form>
<iframe id="userstate" src="about:blank" width="470" height="30" frameborder="1" scrolling="no"></iframe>
The resulting URL produces a simple text string, and has no HTML on the results page, so I feel it should be pretty easy to read this URL as a file, and load the results into an alert box. This would avoid the iframe method, and get out of the mixed content situation. It would run without anyone needing to change anything. But I cannot figure out how to get this to work. I feel like FileReader() should be a good way to do it, but the URL has parameters... so the reader doesn't know what file type it is. Its just failing. There has to be an easier way to do this.

You shouldn't need any javascript for this kind of thing. Just add a name attribute to the username input, and that value will get passed along in the query string as "&username=Name". so:
<form action="/userstate?app=cc">
Username: <input type="text" id="username" name="username" />
<input type="submit" value="Clear User State" />
</form>
That would submit the form to "http://samedomain.wut/userstate?app=cc&username=", which would be available on the server side in the usual post data source.
There are other, better ways of doing this, but I don't know your setup. You should look into server-side management of cookies/session; the user shouldn't have to input anything except to log in initially.
Edit:
If you want to use this as an administration tool, you can do two things: use ajax (XMLHttpRequest or $.ajax from Jquery), or make your endpoint redirect back to (or serve) the form. You could have the form submit to its own url and in your server side scripting process the data, then output the html for the form again. Not a great pattern for actual applications, but it should work in this situation.

Make form redirect users after posting

I need to change a form so that instead of reloading the page after submitting it, it redirects the user to another page.
This is the form I'm talking about:
<form class="questionform" name="questionform-0" id="questionform-0">
<textarea class="question-box" cols="12" rows="5" id="question-box-' . $questionformid . '" name="title" type="text" maxlength="200" size="28"></textarea>
<input type="text" class="ubicacion" value="" name="question">
<input type="button" name="ask" value="Publicar" onclick="askquestion('questionform-0'); window.location.reload(true);">
I want to remove window.location.reload and change it for something that redirects users to the page their comment will appear.
The problem is that it's not simply a static. So I have no idea how to do it. The URL I want to send users to is:
www.chusmix.com/s?=(content of the second field)
How do I do it? Anyway thanks for any info or whatever that points me on the right direction. Thanks

There is no need to use javascript for this purpose. You only need to set the action attribute of the form tag. It tells where the form information will be sent. So in your case it will be:
<form action="http://www.chusmix.com/s">
Also if you want to send the variables through the URL like: http://www.chusmix.com/s?variable=someValue
You need to set the method attribute as get so it will look like this:
<form action="http://www.chusmix.com/s" method="get">
If you don't want the data sent to be visible set the method to post, note that there are different advantages for each method so i recommend you read more about this if this form is an vital part of your webpage.
The variable names that appear in the url http://domain.com?**variable**= will depend on the inputs name <input type="text" name="**variable**" />
For more information on how forms work you can go to:
http://www.w3schools.com/TAGS/tag_form.asp
http://www.tizag.com/phpT/forms.php

You can ad an action:
<form action="redirection_url.php" method="POST" class="questionform" name="questionform-0" id="questionform-0">
I think you can use both absolute and relative url. Also note that I've added
method="POST" - which defines how the data from the form will be sent, as you already send some data with GET method (that's the stuff after ? in your url) - so this should work pretty well.

If you cannot use the action attribute in the <form> tag, you may redirect the user using window.location (you will probably want to do this inside the askquestion method, not in the onclick attribute).
window.location = "http://www.chusmix.com/s?=" + inputValue;