Powering Air Assist using Spindle Relay

I have air assist hooked up to my K40, but I would like to be able to control it from G-code. The control board (Sainsmart 3 axis GRBL controller) has a relay for the spindle (D11 on Marlin, not sure what GRBL uses) that should do the job, but I’m not sure how to map the controls so M7/M8 (or whatever) will turn on/off the air assist. What do I need to change in my GRBL files to make this happen?