You can refer to the following articles:
Enable authentication in your own web API by using Azure AD B2C
How to secure a Web API built with ASP.NET Core using the Azure AD B2C
Besides, in your referred sample, the API application locate in the Service folder, you can refer to the sample code to build your application, pay attention to the package reference and the project dependencies. If you have any issue with referring the sample, just raise the issue on GitHub Issues.
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Best regards,
Dillion