FaceTime with External Camera

I am apparently unable to use my green screen with FaceTime? The iOS15 blurred background portrait mode is an option but would ideally rather use M1 MBA. Elgato suggests using OBS and creating a chroma key filter then “simply” instruct your video software to use the virtual camera for input. I am not seeing any option to use an external video source on M1 MBA running Monterey 12.3.1.