Hey Bob,
you can attach the SSR’s input side to pin 15 (tool-enable) and pin 19 (Ground) on the 25-pin I/O port of the Onefinity Controller.
The Controller will then switch the relay on if you enter “M3 Sx” (“x” is 1 or higher) in the MDI field or M3 is encountered in a program (and will switch it off if it encounters “M5”).
In the Tool Configuration Tab you have to set “tool-enable-mode” to “lo-hi” to get 5 V on pin 15 when “tool-enable” is on.
If pin 15 doesn’t work when entering a “M3 Sx” command in the MDI field, try to set “tool-type” to “PWM spindle”.
By the way this is the same way @satoer switches the router on and off when set to “auto” mode (with the same Onefinity settings).