Calling our extension .ConfigureAuthentication() from the Startup.cs under ConfigureServices

Startup.cs

 public void ConfigureServices(IServiceCollection services)
        {
            services.AddUmbraco(_env, _config)
                .AddBackOffice()
                .AddWebsite()
                .AddComposers()
                .ConfigureAuthentication()
                .ConfigureAuthenticationMembers()
                .Build();
        }