If you try to use Mapbox in the map view in Reality capture you will see this dialogue:
To get the "User token" go to www.mapbox.com, and make a free account.
Then on your account page (https://account.mapbox.com/) find the "Default public token":
This can then be copied and paisted into the "user token" field in RealityCapture:
Then just click "OK" and your Mapbox maps should work.