set_the_usual_form_variables abstracts that process (it also works for URL variables). The code is in {acsroot}/tcl/ad-utilities.tcl.preload.
If it should be possible to call the page without passing in any form variables, you should use:
set_the_usual_form_variables 0
Related commands are export_form_vars, export_url_vars, export_entire_form_as_url_vars, export_entire_form.