Hi:
You may want to work with Tech Support on this question. As far as I know, the only way to use URL= with PDF is to 1) link to an entirely separate PDF document or HTML page 2) link to an internal pointer in the same document.
When you use <FORM> tags in HTML, you are generally executing a program (or a script) which accepts the hidden parameter values as part of the HTTP stream and then the web server performs the form action.
I did not think that you could invoke an HTML <FORM> from within a PDF document in order to make the equivalent of a form action happen. So, this is why I think you have a question for Tech Support.
cynthia