The fetchTokens is an async callback and can start any flow required for the authentication before returning its promise. However, an authentication flow can often involve redirections of entire page. Custom authentication will require you to control the backend that serves your frontend. So there are some options:

1. Let the backend handle authentication prior to serving the application. In this scenario, you will have an authenticated user at the time the application is loaded.

2. Let the application load regardless of whether user is authenticated with your authentication provider or not and let the flow of authentication start with your fetchTokens callback interacting with your server, to 1) fetch a local path on your server that can verify whether user is authenticated or not, and take actions such as responding with some JSON content that code that makes your callback either prompt for authentication in custom dialogs, or redirect the page to a login page and eventually arrive here again once the flow is done.

Both these scenarios are quite similar, just that the second one is prepared for non-authenticated users and take actions to initialize authentication flow.