You can install an IOT Relay to control your spindle using Gcode commands
M3 S1000 ; Turn Spindle On M5 ; Turn Spindle Off
If you would like to use both a Laser, and the IoT relay, See: Connecting both an IOT Switching Relay for a Router, and a Laser, to the BlackBox
NB: Do not plug anything else into the IoT relay (leave the Normally On and Always On sockets disconnected). ONLY connect the Router to the Normally Off socket as shown.