How do I mimic a form with method=post?

I'm modifying my shopping cart.  I used to collect payment data, send it to my payment gateway, read the response returned by my gateway, then display either a receipt or payment declined page depending on what was returned.  Now I want to collect everything but the credit card data.  This requires me to collect everything else, then send my customer to a page on my gateway with the name and address data I collected, as well as the other data my gateway requires, so my customer can enter his/her credit card information.  I can easily send my customer to the gateway page.   Using HttpWebRequest, I can send data to my gateway page.  But how do I send my customer over with the necessary data?  In other words, how do I mimic html's <form action="testResult.aspx" method="post">?