Question

How to get a first access token? Or how to perform the initial authentification using the default client? 

Answer

One does not need to enter any passwords or client ID. The initial authentication is preconfigured. Just open a Swagger page (EU link or US link), click Authorize:

Select all scopes, and click Authorize again:

The password should be specified only for a custom client. A custom client can be created after the initial initialization. 

References

EU Help: Getting a Tenant Identifier or US Help: Getting a Tenant Identifier

EU Help: Creating a Client or US Help: Creating a Client