[Vorph]

I'd suggest you start by doing a "View Page Source" on any page on the forums... or just do a Google Search on this page and look at the GET string it passes to google.

Anyway... the parameter is called "sitesearch" and you just need to put a hidden input element in the form and set its value to the site you want to search.

Edit: there's another parameter called "domains" which is what places the radio buttons for "Search www" or " Search site.com" on the google page you send them to. You may want to set that one too.