RequestBuilder.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApp10
{
    class Program
    {
        static void Main(string[] args)
        {

            var httpClient = new HttpClient();
            
            httpClient.SendAsync(
                CreateRequest(HttpMethod.Get, new Uri("https://bing.com"))
                    .WithAccept("application/json"));
        }

        public static HttpRequestMessage CreateRequest(HttpMethod method, Uri requestUrl)
        {
            return new HttpRequestMessage()
            {
                Method = method,
                RequestUri = requestUrl
            };
        }
    }

    public static class HttpRequestMessageExtensions {
        public static HttpRequestMessage WithAccept(this HttpRequestMessage hrm, string mediaType)
        {
            hrm.Headers.Accept.Add(new MediaTypeWithQualityHeaderValue(mediaType));
            return hrm;
        }
    }
}