M950 P3 C"out9" ; define P3 as mister
Then simply create M7.g and M9.g to turn it on and off...
M42 P3 S1 ; enable mister
G4 P2000 ; wait for 2 seconds
The 2 second wait just ensures that there's active mist/air before the machine moves on.
M42 P3 S0 ; disable mister
It should be noted that I haven't actually built my machine yet, but I've mocked up all of the electronics and relays on a test bench to prove out my design, and the mister does work (at least in a test setting).