I don't have the manual for the tractor to know exactly how they've done the system. One option is that they used a PTO speed sensor to send RPM signal to the dash. The second option, they could calculate the PTO RPM based on the engine RPM since it's a fixed ratio.
The first option, you would need to inspect the PTO speed sensor and the wiring to it.
The second option, there is a switch in the PTO lever for when it's engaged, so it sends a signal to the dashboard and changes the display to PTO RPM. Same situation as before, inspect the wiring and the switch. If the switch is not working, the dash can't know if the PTO is running.
I couldn't find a PTO RPM sensor in the Messicks parts diagram so I'm leaning towards the second option I listed above.