Hello...
I know what you're asking, but it's not /easily/ done. Here are your options:
1) Although a lot of sites specifically request(and use in their examples) FORM, or POSTed variables, they will still accept and parse the URL, or GET variables. Of course, if the information you're sending is not suitable for for URL variables then you will have to continue...
2) Have an interim page on your server that takes URL variables and has a few tasks. First, it does all the db stuffing, etc. Then, it creates a simple HTML page and creates some quick javascript that will redirect the user with the necessary POST data.
3) Otherwise, you'll have to make the request yourself(look for util_http* in the tcl directory) and either send the results back to the user, or hope that you get some sort of sessionid back that you can then give to the user as you pass her off to the third party site.
it all depends on your exact requirements
