Use the full model information
Open the TV's system information and write down the model and platform. Search the actual store with the full app name and verify the publisher. If the store does not offer the app, ask the publisher which compatible route it supports before buying a viewing account.
Test the two screen options
If you are considering an external HDMI box, check its store and account format separately. Run the same authorized item on the setup you intend to keep and verify the remote, captions and guide on that screen.