It is not possible to use a custom marketing form for event registration if you want to enforce portal sign in. This is not a great user experience as it prevents the following
- event attendees being able to return to the portal and manage their event and session registration themselves
- event attendees being able to see that they have already registered for an event (resulting in them creating duplicate event registrations)

Please permit the use of a custom marketing form if allow anonymous registrations is turned off.