Trying to make a simple wordpress plugin, having difficulties understanding how to make it?

by

There are two issues with what your form is currently trying to do:

  1. action="page-rebrander.php" is meaningless to WP (if you actually do have such custom file — you shouldn’t). In WP mechanics you should be typically submitting form to the page itself, options.php (for Settings API), or admin-post.php (for more generic forms).

  2. You are trying to pass data with GET request, but really WP considers anything in URL as something you have meant for it. You should stick with POST for passing data most of the time.