Get Original URL During Unauthorized Redirect

For front end use, we have an SP initiate SSO providing authentication.  When this completes, the user is currently being forwarded to the site's home page.  What we are trying to do is to provide the IDP the originally requested URL so that it they can include it in the relay state.  We would then forward the user to the correct page as the final step.

The problem I am having is getting the originally requested URL when the system forwards the user to the FrontEndLoginPageUrl (set in the back end admin settings).

For whatever reason, the Request.UrlReferrer is not sent by the client.  Is there somewhere the server stores the information so that I can get at it?