i think you mean before T-24? IME, at T-24 or closer to flight, when trying to select a seat from a reservation on the app using my trips, instead of a seat map, it brings up a pop-up that says it’s under T-24 and refers you to check in to select a seat - and you can’t do that with UA when your first flight is with a partner. Only way to do this online is to use the check in option after you are marked as boarded on the last partner flight and only have UA segments left.