As a test I found this position online and played it with both white and black:


Qxa7, Kxa7, bxc8=N

With the engine playing white when the pawn takes the rook the board will flash to show that the promotion is a knight, but note that you must still press the knight button on the board for the under promotion to work. So you lift the rook, lift the pawn, press the knight button and then place the knight on the board.

I realised one issue people might be running into when setting up test positions is that the GUIs send new positions to my driver and my driver tries to work out what move was made. That's how a normal game would progress. But if you are setting up new positions and the new one is almost identical to the old one, but with one piece moved for example, my driver might think that you have actually made a move rather than starting a new game again. Sometimes you need to make a few moves before restarting the position or edit it twice, adding and removing an extra pawn or something in between.