Send token in Rest Api call

I need to send a token when I make this Api call to confirm i am logged in the game .
How can I do it?

public static async Task<HttpResponseMessage> SaveStats(int _monstersKilled, int _score, int _round, string _gameMode, int _userId)
        {
            HttpResponseMessage response = null;
            try
            {
                Stats stats = new Stats
                {
                    MonstersKilled = _monstersKilled,
                    BestScore = _score,
                    HighestRound = _round,
                    GameMode = _gameMode

                };
                string endpoint = "GameStats/Update/" + _userId;
                response = await client.PutAsJsonAsync(endpoint, stats);
                response.EnsureSuccessStatusCode();
                Debug.WriteLine("STATS SAVED for "+_userId);

            }

            catch (Exception ex)
            {
                Debug.WriteLine("\tERROR {0}", ex.Message);
            }

            return response;
        }

   public static void Initialize()
        {
            client.BaseAddress = new Uri("http://localhost:5000/");
            client.DefaultRequestHeaders.Accept.Clear();
            client.DefaultRequestHeaders.Accept.Add(
                new MediaTypeWithQualityHeaderValue("application/json"));

        }