good luck dude. there's a lot more going on on that page than just the form fields that are apparent to you. there are a lot of hidden form input that seem to carry session information and other information that is dynamically created on a per user basis, probably for tracking and statistics.
also, even if you manage to submit something, they probably have some sort of security to prevent random spammers and hijackers (like you) from interfacing into their applications. you might want to look into seeing if they publish some sort of public api for sending text messages...
anyways, give it a shot, but i'm betting that you run into stuff that's a little trickier than just copying a few form elements.