It's the other way round; if you're Brandenburg then there's an event that gives you a PU with Prussia if they have 100+ opinion of you, or they are your vassal. But there's no event to make Brandenburg a junior in a union under Prussia, and I don't think that annexing a vassal gives you their electorship.If you're Brandenburg you're already an electorate. If you're the Teutonic Order and turn into Prussia you can PU/vassalize/inherit Brandenburg; I'm not sure if those also bring along elector status.