How do I add HBO Max to my Apple TV homescreen?
How do I add another streaming service (paid subscription) to my homepage on Apple TV?
What model of Apple TV do you have exactly? Link-> Identify your Apple TV model - Apple Support
If you have a 4th Generation (HD) or later Apple TV, open the App Store app and find the App, then click on the Install button
Link-> Download apps on your Apple TV - Apple Support
If you have a 3rd generation or earlier Apple TV, you cannot add apps. Whatever you see on the Home Screen is what is available to that model.