If you have dual extruders, you can put it instead of the cooling fan, but you'll lose the fan capability and have to use a fan extender to get it back.
If you only have a single extruder, you should have a free port in the screw terminal region there.
Either way, you will have to make 1 or a few small changes in the firmware Configuration.h file.
Remember to put a diode across the relay terminals backwards to prevent electromechanical blowback!
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