Triggers for events: A question I have

How would I make it so that an event is only executed if the player presses the control button? I want it so that when he does this, he gets on his horse( The horse will not be availabe all of the time)

You can't use CTRL, but if you aren't too attached to that specific button, you can use other keys like S, A, Q, W, etc... by setting a parallel process common event which has in it a conditional branch conditioned on a button. Each button corresponds to a key, and you can find out which key by pressing F1 in test play, on the third page.