Q:

[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! 

A:

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">

