If the message app does not show on the watch (it should, both Google and Samsung messages are pre installed), download it. Go to the apps screen on the watch and open the message app of your choice. Before you can send a text, it will ask if you want to make the app you default and will show you a circled "x" and a check mark. Tap the check mark to make it the default and start texting. You only have to do this once.
