[Via ASP.NET Daily Articles]
All of us have had the situation come across where you need to not use the PostBack, but instead have an ASP.NET page POST a form submission to a different page/URL. This article outlines how to do it.
Sometime you need to post a form to an different url from asp.net pages, for example you might need to send user to third party payment processing system using post method, asp.net does not provide any straight forward way to accomplish this task.
Problem which most users faces with server side form in aspx page are, you are not allowed to change action of form and you are allowed to use only one server side form per page.