This option is not yet available for the guest portal. This is only available for sponsor portal. The only option available for now is a customizable FQDN in the IP/NAME part of the URL in your question. The later part cannot be customized for the following reasons.
1. Each portal has it's own portal ID which is unique and this must be included in the redirect URL sent to the WLC/Switch so that the client reaches out to the correct portal.
2. Session ID (pulled from RADIUS session) and the type of redirect are mentioned along with https://IP/NAME:8443/portal/PortalSetup.action?portal=2c050b70-e1d2-11e8-b03e-1c6a7ac127a6 in a guest flow which in case of a sponsor portal are generated right when the user logs in and are stored in the browser cookies instead of sending them in the HTTP request itself.
This limits Guest Portal to not have a unique FQDN as such as it has to tie RADIUS and the Guest user and endpoint sessions. From a development perspective, I see a lot of challenges to come up with a way this can be done but for now, I do not think (or know if) there are any plans as such.
