<p>ASP.NET server-side controls postback to their own page.  This makes cases where you want to redirect a user to an external page, but need to post to that page for some reason (for authentication, for instance) a pain.</p>

<p>An <code>HttpWebRequest</code> works great if you don't want to redirect, and JavaScript is fine in some cases, but can get tricky if you really do need the server-side code to get the data together for the post.</p>

<p>So how do you both post to an external URL and redirect the user to the result from your ASP.NET codebehind code?</p>

If you're using ASP.NET 2.0, you can do this with
cross-page posting
.
Edit: I missed the fact that you're asking about an
external
page.  For that I think you'd need to have your ASP.NET page gen up an HTML form whose action is set to the remote URL and method is set to POST.  (Using cross-page posting, this could even be a different page with no UI, only hidden form elements.)  Then add a bit of javascript to submit the form as soon as the postback result was received on the client.