Got it!
<form action="https://www.yoursitehere.com/myscript/index.php" method="get">
Username: <input type="text" name="user" />
Password: <input type="text" name="pass" />
<input type="submit" />
that form sends the correct input to the script in the form of index.php?user=theiruser&pass=theirpass
I still want to figure out how to do it dynamically…
Should I use cookies or session or what?