RC9_function missing. How to use the AUX?

I have two copters that I wired up the same and one I have the camera switch working on rc8 by using rc8_function = 1

On my other copter which has 3.5.1 I have more channels so naturally I want aux 1 (rc9) to be the switch but there is no rc9_function. I checked on RC8 and there too is no function to set it to passthru

How do I set RC9 to passthru out on aux 1 when I have no rc9_funtion?

SERVO9_FUNCTION and choose an option.

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