Love seeing someone doing casts of AoM content, I hope that you keep doing these with Retold! I'm not at all experienced with AoM as a player, I just used to play it as a kid, but I'd love to be able to learn about the game and do content like this
If you use Ippert's UI, there should be a button in the top right labelled OBS for observer mode. I believe that is it, though I've never personally used it so can't be 100% sure.