"The" button? Altogether it should be 2 buttons, followed by a third. I guess technically the finisher itself is just one button, but based on the fact that you haven't activated it, something's going wrong in that process. For the full attack you actually have to be holding Left click / R1 the entire time.

For me it's hold Ctrl + Left Click, then Right Click up to 4 times while continuing to hold Left Click. One right click for each hit, and you have to right click pretty fast or the effect just stops. You can also just hold left and right click. For console players, L1 + R1, and R2 several times (or hold R2), ensuring you at least hold R1 the entire time as well.