It’s not any different. When you play the in full keyboard mode, set the chord recognition appropriately ie full keyboard or AI full keyboard. The vocal harmony will then work as normal.
However, you do have to be playing in a way that allows the system to identify a correct chord. There is no way round that, the system has to be able to identify chords from what you play to work reasonably. AI helps, but you as the player still have to give it enough data to work on.
Mike