Phil nailed it!
From the
3520 online manual, item #7:
1. Sit on operator痴 seat. 2. Move transmission to neutral position:
PowrReverser: Depress clutch pedal completely and move the transmission gear and range shift levers and the reverser lever to the N (neutral) position.
eHydro: Move the range shift lever to the N (neutral) position.
3. Lock park brake.
4. Push all PTO engagement knobs to the disengaged/off position.
NOTE: The starter will not crank if the rear/mid PTO engagement knob is pulled to the engaged/on position.
5. Start the engine and adjust speed to 1500 rpm.
6. Move the PTO selector lever to top/rearward position for rear PTO only.
7. Get off the operator痴 seat.
8. Pull the rear PTO engagement knob to the engaged/on position to engage the rear PTO.
The engine should continue to run.
The instrument panel PTO engaged light will illuminate when the rear PTO is engaged.
9. Adjust the hand throttle lever forward to the desired speed for implement used.
NOTE: The PTO marker on the tachometer indicates engine speed for a standard 540 PTO.