HOW TO IMPLEMENT A TIMING KEY PRESS EVENT

I'm trying to implement an event where (take for example) you have a bar that goes from left to right. On the bar it has a small green area while the rest of the bar is red. Then you also have a marker moving from left to right and you have to press a certain key while the marker is in the green section to make the event work.

I'm using RM 2K3 btw. I tried using pictures to simulate it and got up to having the marker move along the bar. I just don't know how to make it so that you can press a certain key at the right time.