Looks like the panel had a option for Spindle Override on some versions.
Do you have the control documentation? Generally most CNC systems have a dual controller, one for the CNC (motion) and one for the PLC (M,S,T codes) , the PLC usually has an output to turn on the spindle, at minimum.
If it does have a M3, M4, M5 even if no S command , you may be able to use this for the VFD.
Al.