If I iframe a website that has a signup form, and under the iframe (on my webpage), could I put a button/link that would "submit" the form that is in the iframe? Sorry if this doesn't make much sense... Basically trying to do what people do for CPA Co-Reg offers.
If the form in the iframe is coming from another domain, no. Browser security will prevent access to the framed document.
In which case use curl to snatch the code and present the other form as if it comes from your site. then you can use the javascript.