Form action=”what ?” (post)

I have to re-use a contact form with some speficic inputs in a new WP site.

I tried :
– insert php code into a classic page.
– write a template specific to this form

– form action=””
– form action=”urlofmysite/contact-form/”
– form action=”urlofmysite/?page_id=10″

Anyway, I got an “sorry no posts matched your criterias” everytime I submit the form.

Have I to write or hack a form plugin to match my needs or is there a way to get a form working on a WP page ?