Using a URL to Post to a Form

Ok this isn't directly wordpress related right away but I hope to make a plugin once I figure out this URL structure. The goal will be to use Gravity Forms to register someone for a webinar, in this case I am using Meeting Burner.

I have the start of it figured out, but I can't get it to submit automatically yet.

The webinar registration page is at

http://webevent.enterthemeeting.com/m/DRBU69VD

And what I have so far to figure out filling the fields works, but I would like the link to submit and basically just go to the thank you page as soon as it submits.

http://webevent.enterthemeeting.com/m/DRBU69VD?f_firstname=Test&f_lastname=User&f_email=adam%2Btest2%40adamteece.com

Any chance I could get some help with figuring out how to sibmit via URL. I know something similar works with GoToMeeting.

Thanks so much for your help.