From the TV's Home screen, navigate to and select APPS, and then select the Search icon in the top-right corner. Next, enter the app you want to download and select it. You will see detailed information about the app. From here, select Install.
Access to new apps will be occasionally added to your smart TV through software updates.
Note: Only apps available in the App Store can be installed on the smart TV.
If the App that you are looking for is not in the App Store I recommend reaching out to the developer.
Whats the reason why it doesn't work for earlier model, such as my 2015 model. I have HBO Go but its being replaced with HBOmax. Is it just because of the available memory?