How can we improve WHMCS?

Some external links are to login-protected areas in WHMCS, see the example below:

https://hosting.encirca.com/viewinvoice.php?id=8108

WHMCS redirects the user to the login template (login.tpl), and even if SSO integration, the previous location is lost.
I tried to modify JS in login.tpl using document.referrer.

The problem is, since the first link to the invoices is intercepted by WHMCS, that information is lost.
I noticed, though, that WHMCS adds a cookie at that point.

We need to recover it in the SSO code, so that we can do a custom redirect back to the original link

WHMCS stores the original link in a variable (perhaps session vars), or in the URL parameters.