Here is the entire file.

using Newtonsoft.Json;
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Linq;
using System.Net.Http;
using System.Text;
using System.Threading.Tasks;
using m2m.Model;
using System.Runtime.CompilerServices;
using System.Net.Http.Headers;

namespace m2m.ViewModel
{
    public class PromoDetailsModel : INotifyPropertyChanged
    {
        
       // public List<PromoDetailsResult> Promoparent { get; set; }

        private List<DetailsHeader> header;
        public List<DetailsHeader> Header
        {
            get { return header; }
            set { header = value;
                PropertyChangedEventHandler handler = PropertyChanged;
                handler?.Invoke(this, new PropertyChangedEventArgs("Header"));
            }
        }


        public async void GetPromoDetails(int id)
        {
            var url = string.Format("http://tmad.us/api/v1/PromoDetails/getDetails?id={0}",id);
            await getPromos(url);
    }
       
        private async Task getPromos(string url)
        {
            //using (HttpClient client = new HttpClient())
            //{
            //   // DetailsHeader test;
            //    client.BaseAddress = new Uri(url);
            //    var response = await client.GetAsync(client.BaseAddress);
            //    response.EnsureSuccessStatusCode();
            //    var JsonResult = response.Content.ReadAsStringAsync().Result;
            //    var promotemp = JsonConvert.DeserializeObject<List<DetailsHeader>>(JsonResult);
            //}

                        using (var client = new HttpClient())
                        {

                            client.BaseAddress = new Uri("http://tmad.us");
                            client.DefaultRequestHeaders.Accept.Clear();
                            client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
                            HttpResponse response = await client.GetAsync(url);

                            if (response.IsSuccessStatusCode)
                            {
                                var result = await response.Content.ReadAsAsync<DetailsHeader>()
                            }
                        }
            


        }

        public event PropertyChangedEventHandler PropertyChanged;

        private void NotifyPropertyChanged([CallerMemberName] string propertyName = "")
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }
    }

}

>Is using System.Net.Http; at the top of your .cs file?
>
>>Yes. Here is an image of what I am seeing.