The easiest way to do this is to customize the ULP. Either new or classic ULP is very customizable.
If you cannot achieve what you need that way, you can do this. You need to disable self-registration in the DB connection, host your own registration page with a back end that can call the management API and create the user (and put a link to that page on the ULP). The user is not automatically logged in with this approach, as they are if you do everything in the ULP.
John