My experience is with RAMPS. Hopefully someone here can help.

Ultimately, you need the firmware configured to output a servo signal on one pin and arrange for 5V and GND on the two others. Some boards, like RAMPS, have provisions for the power.

Look through the firmware and see what it's doing when you set to 34. 34 normally changes the fan output to be a second extruder heater and moves the fan to another pin. Sometimes it's D11, so make sure you aren't trying to put the fan and the servo on the same pin. The fan output should be straight-up PWM, and a servo will look like a 1-2ms pulse every 22ms. You should be able to find the servo outputs with an oscilloscope, if you have one, but be aware that the signal may go away if you have SERVO_DEACTIVATION_DELAY set.