There's a newer version of this sample. Check it out: Azure AD B2C: Call a .NET web API from a .NET web app sample
If you really need to access this sample, you can navigate to the master branch, but please know that it's no longer maintained.
AzureADQuickStarts/B2C-WebApp-WebAPI-OpenIDConnect-DotNet
A .NET MVC web application code sample, secured by Azure AD B2C, that calls a Web API
C#Apache-2.0