Hold down shift and ctrl - then go to the Modify menu then release the mouse button, (when positioned over the Center Pivot command). The short cut will appear on the shelf that is currently active - so it's best to have the shelf you want the short cut in to be set first.

If you select the wrong command, by mistake, you can middle mouse button drag the short cut into the bucket (above right of the shelves). You can also reorganise the shelf buttons my middle mouse button dragging them arround, inside the shelf.

REMEMBER to save your shelves before quiting Maya.......