ref – http://stackoverflow.com/questions/39419854/xcode-8-code-signing-error

When you run a iMessage default app, you will notice three icons next to the text message text field. A camera, digital draw, and the iMessage app icon.

Once you touch the iMsg app icon, it will bring up a list of iMsg apps. You will see your app testMessage like so:

Once you select your app, you’ll see it being run. You will see the “Hello World” label, along with the name of your app.

Then, once you click on the arrow button on the bottom right, you make the iMsg app run full mode