This is absolutely annoying. When the new mark ability was introduced to me, the inquisitor got nothing, instead, Dorian and Blackwall got the new ability (of course it is useless at the last moment in final boss fight), so i'm sticking at releasing the last hit when companions keep shouting at me without any hope. BIOWARE, you should simply let player finish the boss like closing a rift in stead of needing any specific ability.
I feel your pain. I had no idea that it was mappable at all. It shows in the menu, but there is no way to map it to a control. I'm on PS4, on my second playthrough. During my first, it didn't map either. I couldn't understand why my companions kept telling me to discharge something I had no control over. I have no idea how I got through the final battle. Hopefully someone will know something! Ugh.
MamaThyia, could you tell me how you solve your problem even it may not like the same.
After searching Answer HQ, seem like some people have same issue like mine.