IHttpClientFactory in .NET Core 2.1 Console App references System.Net.Http

The official documenation suggests that all I need to add to my project is a reference to the Microsoft.Extensions.Http NuGet package. I've done this.

That's true but in order to make things easier, you have to add Microsoft.Extensions.DependencyInjection as a NuGet package, in fact, you can delegate all the creation of httpClient instance to the HttpClientBuilderExtensions which add a lot of extensions methods to create a named or typed HTTPClient here I have written an example for you

using System;
using System.Net.Http;
using System.Threading.Tasks;
using Microsoft.Extensions.DependencyInjection;


namespace TypedHttpClientConsoleApplication
{
    class Program
    {
        public static void Main(string[] args) => Run().GetAwaiter().GetResult();
        public static async Task Run()
        {
            var serviceCollection = new ServiceCollection();


            Configure(serviceCollection);

            var services = serviceCollection.BuildServiceProvider();

            Console.WriteLine("Creating a client...");
            var github = services.GetRequiredService<GitHubClient>();

            Console.WriteLine("Sending a request...");
            var response = await github.GetJson();

            var data = await response.Content.ReadAsStringAsync(); 
            Console.WriteLine("Response data:");
            Console.WriteLine((object)data);

            Console.WriteLine("Press the ANY key to exit...");
            Console.ReadKey();
        }
        public static void Configure(IServiceCollection services)
        {







            services.AddHttpClient("github", c =>
            {
                c.BaseAddress = new Uri("https://api.github.com/");

                c.DefaultRequestHeaders.Add("Accept", "application/vnd.github.v3+json"); // GitHub API versioning
                c.DefaultRequestHeaders.Add("User-Agent", "HttpClientFactory-Sample"); // GitHub requires a user-agent
            })                        
            .AddTypedClient<GitHubClient>();
        }
        private class GitHubClient
        {
            public GitHubClient(HttpClient httpClient)
            {
                HttpClient = httpClient;
            }

            public HttpClient HttpClient { get; }

            // Gets the list of services on github.
            public async Task<HttpResponseMessage> GetJson()
            {
                var request = new HttpRequestMessage(HttpMethod.Get, "/");

                var response = await HttpClient.SendAsync(request).ConfigureAwait(false);
                response.EnsureSuccessStatusCode();

                return response;
            }
        }

    }

}

Hope this help