In short, if you're using a modern browser like chrome, safari or Firefox, you would use vanilla javascript to find the form elements you care about, edit their values, and then submit the form. The function you'd use is document.querySelectorAll().
Example:
Code:
document.querySelectorAll('form#login input#username')[0].value = " *YourUserName* ";
document.querySelectorAll('form#login input#password')[0].value = " *YourPassword* ";
document.querySelectorAll('form#login')[0].submit();
// or document.getElementById('login').submit();
in order to execute this safely from the URL bar, you should wrap it in a closure.
Code:
javascript:(function(){ ... })()