1. AUX1 channel basic setup on Pixhawk

2. Hardware connection
5V from UBEC( to any free connector)
Servo to AUX1

3. MicroSD setup
Create a folder etc with extras.txt file inside( Check carefully file extension, in windows you can create by error file extras.txt.txt)
Into file put following lines fmu mode_pwm mixer load /dev/pwm_output1 /etc/mixers/pass.aux.mix
Put SD card into Pixhawk slot 

4. Pixhawk setup via QGC Connect to your Pixhawk
Go to Parameters, and using Channel Monitor check if you can operate channel 7 from RC transmitter
Next go to Radio Calibration section in Parameters group
Set value 7 to parameter RC_MAP_AUX1 and if needed setup channel parameters:

5. Operation
Power ON in following sequence 1. RC transmitter 2. Your copter 3. Start QGC
Press Safety Switch till it starts to blink twice and you have a confirmation song.
Now you can operate connected servo from RC transmitter