It’s mentioned in the article in the generating access token section. Basically, this should be part of the server code, you will need to create a server solution that can fetch access tokens using the Power BI .NET SDK or REST APIs. The Angular client can then call the access token fetch API first and then use this access token to call the embedding API. The sample server solution is here: https://github.com/Microsoft/PowerBI-Developer-Samples/tree/master/App%20Owns%20Data