As of this writing the token auth flow is broken due to the following reasons:
- The country based override causes failure of the "Activate in iTunes" button to show up.
- Once the button exists, it attempts to open the Music app rather than the Books app.
Before attempting any of this, go to Audible library and download your audiobook and open it, it will get added to Books app on macOS. Now try to play it and it will as for permission to authorise it.