How to add HTTPS REST value at Registration?

After registration through Community Builder I need to send a set of values from the registration fields to a master database. The master database is going to take those values and provide an ID back in a JSON format. We then need to pick up the provided ID from the master database and assign it to a field for that user’s profile. This all has to happen before the user is redirected to their newly created profile.

What is the best way to achieve this goal? If it is of any significance we are also using CB Subs for monthly subscriptions that the user must activate at registration.

Only way to do that is new CB plugin to act on CB triggers or use CB Auto Actions and its Code action with Method set to PHP then use PHP to do all you're wanting. I suppose there's other ways to go about it, but it's going to take coding regardless.