public class Session
    {
        private readonly static Lazy<Session> _instance = new Lazy<Session>(()=>new Session());
        public static Session Instance { get { return _instance.Value; } }

        private Session()
        {
        }

        public string Token { get; private set; }

        public void SetToken(string token)
        {
            Token = token;
        }
    }

    public class Test
    {
        public async Task<string> Post(string uri, string data)
        {
            var request = (HttpWebRequest)WebRequest.Create(uri);
            request.Method = "POST";
            request.ContentType = "application/json";

            var buffer = Encoding.UTF8.GetBytes(data);
            request.ContentLength = buffer.Length;

            var stream = await request.GetRequestStreamAsync();
            await stream.WriteAsync(buffer, 0, buffer.Length);
            stream.Close();

            var response = await request.GetResponseAsync();
            stream = response.GetResponseStream();
            if(stream == null)
                throw new ArgumentNullException();

            var reader = new StreamReader(stream);
            var result = await reader.ReadToEndAsync();


            reader.Close();
            stream.Close();
            response.Close();
            return result;
        }
    }