how do I post info to form on replicated site

there are replicated sites for every broker on this frontpage site.  the mother sight has the form and whenever they click on a pay button their needs to be some code sent with it to fill in a hidden form field on the form on the mother site that it links to.  any help?  any way I can do this with HTML, I have to put the code in on 1000 broker's sites so i need to be easy as possible. thanx ??? ???