You can get the Harmony activity to start with @ogiewon ‘s driver found here
You can, with careful creation of a rule using custom actions, manipulate the harmony interface and select an item. But you would need to make sure the Apple TV always returns to menu first, and goes to a specific location before moving to your video app and opening it. In other words you can only do this with button pushes, not direct communication to the Apple TV. The driver isn’t going to know where it is, it’s just going to press buttons in the Harmony interface. Since Apple TV only sleeps it never actually turns off, when you press the menu button and hold for example to return to the main menu, it doesn’t always go back to the same location on the screen. It returns to the app you were last using.
So chances of this working correctly all the time will require the selection of a “home” position first.
Another thought is to instead use Harmony Alexa skill (The second one - there are two). With that skill, you can often say things like “Alexa, tell Harmony to....” and you add the command. There are favorites options on Harmony, but I’m not sure they work for anything other than TV channels ( That’s all I’ve ever used them for). You might be able to use one of those. They are added as scenes to the Alexa app if I remember right, and the Harmony Second remote Alexa app can control them.
If that works by voice, then you might be able to control it with an Alexa Routine custom command.
