Umbral finisher

Okay so I have scoured the internet and for whatever reason I can’t find an answer on exactly how to do an Umbral finisher lol. Like I understand most of it like I know you have to soul flay and then deplete their white bar but what exactly do you do after that? I can’t find out what button to press when I go up to them to do the finisher. It doesn’t even say in the tutorials in the game itself. I tried going up to them and pressing L2 (I’m on PS5.) to pull out the lamp but it doesn’t trigger the finisher. Unless I’m not positioned just right?