How to use a key to enable/disable an overlay?

First, I don't know exactly the difference between a dashboard and an overlay. I suppose I need to use an overlay, because what I want is just as simple as an image in the middle of the screen (it will show a description of the functions of my button boxes). I have already created the overlay with the background image I need. Then I went to controls and events, controls, new mapping, but then I cannot see a way to enable and disable my overlay among the hundreds of options. Is it possible? Oh, btw, my overlay must be visible in virtual reality, in the main display of my headset. Is it possible or am I going the wrong direction? Thanx.