If you are the emperor then it needs to be inherited. This can either occur on succession 50 years+ after forming a pu or as a result of the rare direct inheritance succussion. Suggestions:

1. As Austria, your best path to electorship is forcing a pu with bohemia using the mission cb.
2. You cannot make a subject an elector so if you have a free elector slot try giving it to someone who you are likely to form a pu with (ideally someone who you already have or can receive through a mission, a restoration of union cb for such as Bavaria if you're Austria and it exists). If none exists try to find a member with an old ruler and no heir and/or your current dynasty.

NOTE:

1. integrating a pu will NOT give you the elector title.
2. For some reason, when you inherit a pu, for some reason you will not vote for yourself in that specific election, so if you anticipate inheriting, I suggest making sure you have at least 3 other supporters.