Hey Steve,
The spindle start command is M3. It will send the RUN command to the VFD (if connected correctly), except if S is set to 0, in which case it will do nothing.
By the way, as far as I have seen until today, all spindles with max. 24,000 rpm @ 400 Hz must run at least at 6000 rpm. 1000 rpm is too low. In my spindle manual, the spindle operator is told to set this as a minimum limit for the frequency in the VFD settings to prevent the spindle running lower.
The frequency calculates this way.