For macOS, you can use either the loopback interface (where AppAuth will generate the redirect URI for you), or a custom scheme. To create a custom scheme redirect URI, reverse the client id to get the URI scheme, for example com.googleusercontent.apps.IDENTIFIER and, add your own path component. E.g. com.googleusercontent.apps.IDENTIFIER:/oauth2redirect/google. Note that there is only a single slash (/) after the scheme.