Configuring the front-end Web application for identity delegation

Question

I have read the article in this link and it describes the exact scenario I am trying to acheive.

I am trying to achieve ADFS Impersonation as described below:

The ADFS settings have been applied, how do I enable an ASP Core API to accept the credentials? My ASP Core API is already Windows Auth (Negotiate) enabled. Extension snippet below:

/// https://docs.microsoft.com/en-us/aspnet/core/security/authentication/windowsauth?view=aspnetcore-6.0&tabs=visual-studio
        /// Install-Package Microsoft.AspNetCore.Authentication.Negotiate
        /// </summary>
        /// <param name="services"></param>
        public static void ConfigureAuthentication(this IServiceCollection services)
        {
            // https://stackoverflow.com/questions/63150369/how-to-config-addauthorization-on-net-core-in-order-to-allow-roles-for-multiple
            services.AddAuthentication(NegotiateDefaults.AuthenticationScheme).AddNegotiate();

            // replace the handler
            var serviceDescriptor = new ServiceDescriptor(typeof(NegotiateHandler),
                                                          typeof(NtlmNegotiateHandler),
                                                          ServiceLifetime.Transient);
            services.Replace(serviceDescriptor);

            services.AddAuthorization(options =>
            {
                options.FallbackPolicy = options.DefaultPolicy;
            });
        } 

The solution in the article is this:

client code

This is my code on the front-end connecting to the back end service.

HttpClientHandler handler = new HttpClientHandler(); 
handler.UseDefaultCredentials = true; 
handler.Credentials CredentialCache.DefaultCredentials; 
handler.PreAuthenticate = true; 
HttpClient client = new HttpClient(handler);

I'm getting 401 Unauthorized on an API which should allow access.