How to trigger power off command with Logitech Harmony Hub on an AppleTV
By default, there is no action to trigger the power off of an AppleTV from Logitech's Harmony Hub. But there is another option to set up a different device for doing that.
Create a new device called "Harmony Pro" and type in the serial "BT Apple TV 4". This creates an AppleTV device within the Harmony app with a quite different configuration. If you use that device in your actions, the hub will sent a power off/sleep command to the AppleTV.