If the app keys do not work on your source device (which may be the case if your original remote did not have keys like this), you can create a live macro that will access the app for you.
For example on an Apple TV pressing the Home key followed by two presses of the Menu Right key and then the Enter key, will open Netflix. A live macro will allow you to assign all of these key presses to one of the app keys, so that every time you press it Netlfix will open.