ASP.NET Retains POST data through login redirect

I have a site (SiteA) using ASP.NET Membership. One thing it has to do is let the end user on a separate site (SiteB) launch a new browser window on SiteA page with the POST data attached to the page preload.

If the user is registered with SiteA then launching with SiteB works and the page is pre-populated. However, if the user is not logged in, SiteA authentication is first redirected to the login page. After logging in, the user is redirected to the last page, but the POST data is lost and the form is not filled.

How can I save the POST data to the last page?