If you don't need the PW API to process the form, you can place your script in root.
action="<?=$config->paths->root . 'myscript.php'?>"
Although, as already mentioned, I'd suggest to create a PW page + dedicated template for such stuff. Together with URL segments, you can place all your form processing logic in one single file (or even include() each sep. form processing logic, in case there's many of them). Well, actually URL segments aren't really needed, you can just as well include a hidden field in your form, or add a ?form=support to your action URL. Many ways to skin a cat ?