Xamarin-RestAPI.cs

using System.Net.Http;
using System.Net.Http.Headers;

using System.Collections.Generic;
using System.Threading.Tasks;

using Newtonsoft.Json;

namespace BytesCrafter.Xamarin
{
    public class User
    {
        public string uname = string.Empty;
        public string pword = string.Empty;
    }

    public class Token
    {
        public string snid = string.Empty;
    }

    public class RestClient
    {

        HttpClient client;
        string restUrl = string.Empty; //https://www.example.com/api + /user/auth

        public RestClient(string rootUrl)
        {
            restUrl = rootUrl;
            client = new HttpClient();
            client.MaxResponseContentBufferSize = 256000;
            client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/x-www-form-urlencoded"));
        }

        public async Task<Token> SignIn(User user)
        {
            var postData = new List<KeyValuePair<string, string>>();
            postData.Add(new KeyValuePair<string, string>("UN", user.uname));
            postData.Add(new KeyValuePair<string, string>("PW", user.pword));
            var content = new FormUrlEncodedContent(postData);

            var response = await PostResponse<Token>(content);
            //More checking about the response and request here.
            return response;
        }

        public async Task<T> PostResponse<T>(FormUrlEncodedContent content) where T : class
        {
            var response = await client.PostAsync(restUrl, content);
            var result = response.Content.ReadAsStringAsync().Result;
            var token = JsonConvert.DeserializeObject<T>(result);
            //Try catch if result is an actual json.
            return token;
        }
    }
}