Nope, nothing special, let's say your Tony Ferguson, and you knock them down into the side control foetal FTF position, when you jump on them to strike, the grapple HUD will pop up with Armbar<- * ->Arm Triangle.
You don't need to press anything. Just jump on them, and it'll be there if your in the right position and the fighter has the sub.
I press strike once to engage into the FTF. nd while that animation is playing I'm literally hold RS+Left which will make you jump directly into the armbar before the prompt even appears.
If you have two controllers. Use T-ferg and keep dropping the other fighter until you get a severe knockdown (the one where you can jump on them to finish with strikes) and jump on them and the prompt will appear.
Maybe you just haven't got the right position? Dunno, but Tony 100% has those two from the Side Control Foetal position (for lack of a better positional name haha)
Hope this helps.