As I understand it, and if it is like Mach 3, then yes - you'll need a feedback loop. I don't know the specifics but in the Spindle configuration there should be a check box for turning this function on and then some P, I, and D settings to fine tune for stable operation. You'll need a pulse input from a synchronizing system (usually a disc that spins with the spindle and has 1 or more slots that are detected by a sensor) to input these pulses on an input pin configured for this purpose. Then you can have the RPM DRO show the RPM calculated from this feedback system.
I've never done this so I'm not sure of all the details. Have a good read through the manual. There should be a section in the configuration chapter that goes over this.
Stephen "Highspeed" Kruse