do you have a link to the manual
or a clear photo of the control board showing the lables printed on the board ?
my first thought is to either connect the three G540 PWM terminals
to DC12-60 Spindle Controller potentiometer like this
just make sure the maximum voltage across the DC12-60 Spindle Controller potentiometer terminals
is no more than 12V
G540 PDF manual
G540 REV8 Manual.pdf
from the G540 PDF manual page 4
"ANALOG OUTPUT: This is a 0V to +10V opto-isolated analog output intended for use with VFD drives. VFD OUT goes to 0VDC while the G540 is disabled. Connect VFD GND, VFD OUT and VFD +10V to the VFD drive inputs. Make sure the VFD drive positive voltage does not exceed +12VDC. Do not short VFD OUT to any other terminal. Do not reverse polarity to VFD GND and VFD +10V or the G540 may be damaged "
or
a G540 output and a 5V power supply to the DC12-60 Spindle Controller GND & PWM
John