How do I go about asking/informing my landlord that I'd like my partner to move in?

Usually every lease says you must get permission to move someone else in. If you break your lease you have broken the law and you can be evicted. That will be problematic when you apply for new residences.

Be honest and upfront and there should be no problem. Simply say due to educational or work schedules your partner cannot join you until whatever month. Give your partners information so both of you can be background checked at the same time.