yes that's the the ground terminal I am referring to
and the +5V next to it will be needed to power the C3 board
if the FC350 speed control board can supply enough power to supply the C3 board +12V in
this would work for CNC control only
note - a relay will need to connect the FC350 board terminals 3 & 5 to start the motor
or if you want to switch between manual and CNC control
if the FC350 speed control board can not supply the 12V supply for the C3 board
you will need to add an isolated 12V supply