On an Arduino Uno, the GRBL Probe input is on A5. I suspect this board follows the Uno pinout which makes it pin 28 on the processor on your board. Unfortunately it doesn't seem to have a probe pin connector. So much for "Pro". You could try using a DMM to find if pin 28 goes anywhere.
By the way, the standard woodpecker board (which does follow the Uno pinout) has an unlabeled pin that is the probe input. Are you sure the board pictured is the board you have?
Also, what are the "Steering Gear" pins for?