According to this video it should be quite easy
No matter how many times I tried I cannot get anything to respond when connected to the AUX pins on my Pixhawks.
I ended up going around it the long way
I use a 10 channel RX in PWM mode, connected to the pixhawk with a PWM/PPM converter which takes care of the first 8 channels for the Pixhawk to use and then connect my servos direct to Channels 9 & 10 on the RX.
A bit Heath Robinson buy at least it works
Unlike trying to persuade the Pixhawk to pass channels 9 & 10 to the AUX Pins
Has anyone actually managed to get a servo working on the Aux pins, just to confirm it should be possible, or has the option been removed or broken in the latest updates.