Switching Tuner Sources in HomeKit

Configuration
1

I’m new to Home Assistant but I’ve got most of my devices up-and-running (via a docker container), but I’m stumped by one last thing.

I have my Onkyo home cinema amp in configuration.yaml (with just two sources):

media_player:
  - platform: onkyo
    host: 192.168.4.200
    name: amplifier
    sources:
      cbl: "Cable"
      bd: "Apple TV"
      
input_select:
  source:
    name: Amplifier Inputs
    options:
      - Cable
      - Apple TV
    initial: Apple TV

and under homekit:

        - media_player.amplifier
        
        
    entity_config:
      media_player.amplifier:
        feature_list:
        - feature: on_off

This gets me an on|off switch in HA and HomeKit, together with a source selector in HA (which shows as a couple of switches in HomeKit). Power works fine, but nothing happens when I select an input. I suspect I need a script or automation or something, but that’s why I am here. Help!