Thanks for the answer guys. Both were useful. It's a contest so I agree it should go to CRM. But in this case, what I will do is create a form on an external service like you suggested and embed that form into my Marketo page.
Regarding the report, I managed to get only the report I want using Smart List: and choosing Filled Out Form > Form name is > Name of my form. But this report shows me the Total of Leads that filled out that campaign. How do I list those result fields?