When you "call" the page it will be the target of a new HTTP request. grep around for uses of "return_url" to see how this situation is normally handled.
set return_url [ad_conn url]?[ad_conn query]
will get you a url that references the current page. Pass it using export_vars if you're generating a hypertext link or in the -export clause to ad_form if you're building a form that needs to include the return address.
