Exemplo de criação de usuários

Program.cs

using IdentityModel.Client;
using Newtonsoft.Json;
using System;
using System.Net.Http;
using System.Text;
using System.Threading.Tasks;

namespace Client
{
    public class Program
    {
        public static void Main(string[] args) => MainAsync().GetAwaiter().GetResult();

        // <PackageReference Include="IdentityModel" Version="2.12.0" />
        
        private static async Task MainAsync()
        {
            var disco = await DiscoveryClient.GetAsync("http://localhost:50751");

            var tokenClient = new TokenClient(disco.TokenEndpoint, "clientcredentials-qa", "secret");
            var tokenResponse = await tokenClient.RequestClientCredentialsAsync("admin-api");

            if (tokenResponse.IsError)
            {
                Console.WriteLine(tokenResponse.Error);
                return;
            }

            Console.WriteLine(tokenResponse.Json);
            Console.WriteLine("\n\n");

            using (var client = new HttpClient())
            {
                client.SetBearerToken(tokenResponse.AccessToken);
                client.BaseAddress = new Uri("http://localhost:50751/extensions/v1/");

                var body = new StringContent(JsonConvert.SerializeObject(new
                {
                    FirstName = "Nome",
                    LastName = "Sobrenome",
                    PhoneNumber = "Telefone",
                    UserName = "email@dominio.com.br"
                }), Encoding.UTF8, "application/json");

                var response = await client.PostAsync("users", body);
                if (!response.IsSuccessStatusCode)
                {
                    Console.WriteLine(response.StatusCode);
                }
                else
                {
                    Console.WriteLine("OK");
                }
            }

            Console.ReadKey();
        }
    }
}