You can't easily force a browser to POST data to another site.

You might be able to work something out with JavaScript. I have no idea if it would work but you might be able to send back a form with the action and method appropriately set, all of the data you want to send stuck in hidden inputs, and then submit the form in the onLoad method. Personally, I'd avoid your site like the plague if you tried pulling those kinds of shenanigans but I might not be representative of your user base.

A better idea would be to make the request on their behalf (using LWP::UserAgent, for instance) and then send the result back to the client.

-sauoq
"My two cents aren't worth a dime.";