The easier way should be to add an output in your mixer file. A quadrotor mixer usually configs the first 4 pwm outputs, so if you want to use the 5th one, you need to add an extra line setting up a simple mixer https://dev.px4.io/en/airframes/adding_a_new_frame.html#mixer-file.
In your code, you must then publish into
actuators_control topic the servo output that you want.
The same works for aux ports, nonetheless, you must config the auxiliar mixer file.