Use of WWW:Mechanize, suggested above is certainly good advice but take heed of cbrandtbuffalo's note. Many, many webmasters try to offload some of the cost (server cpu cycles, bandwidth when a required field is not filled in, etc) of the form validation (or even untainting, which is spectacularly UNwise) onto the client, via javascript. So you need to be prepared for problems.

And forgive me if this is excess-suspicion, but your words

"..to populate an existing form of a third party vendor"

may not mean what I fear they might, but if you're talking about using that vendor's bandwidth, etc, for your own purposes, you're talking "rude," to saying nothing of "actionable!"

In fact, in the US in this post 9/11 era, that might even be criminal.