The URL for the captive portal on the PAN firewall follows this format:

https://{{ Redirect Host }}:6082/php/uid.php?vsys=1&rule=0

:6082 is the listenning port for HTTPS, :6081 would be for HTTP.

The ?vyss=1&rule=0 is important as well - it tells the firewall which Authentication policy and vsys to reference on the firewall. If you leave these values off, the firewall will throw a 500 Internal Server Error