Open a new browser tab and visit the Authorization URL returned by the InitializeOAuth Choreo. When prompted, log in as your Power BI user and grant access, you will redirect to the Reply URL.
Now run the Microsoft > OAuth > FinalizeOAuth Choreo, specifying the Callback ID returned earlier by the InitializeOAuth Choreo. You'll also need to include the same Resource as above.
At this point, you'll be able to use your Client ID, Client Secret, Username, and Password to access your Power BI account via the API.
Note: if you are building an application that needs to access Power BI accounts for multiple users, you'll need to pass the Access Token returned by the FinalizeOAuth Choreo to authenticate your application's users. You can use the Refresh Token with the Microsoft > OAuth > RefreshToken Choreo when an Access Token expires.