HTTP-Authentication add token to Client

Question

I could manage to get the API-Server sending me a Token. Also i can store the Token in a CustomModel. But How can i attach the Token to my HttpClient which i want to use.

Here's my Login and function:

public static async Task<AuthToken> Login()
        {
            var userName = "administrator";
            var passwd = "********";
            var ClientID = "acef4bf9988848a0bff79485d1ac2648";
            var ClientScrt = "********************";
            var url = "https://****.********.****/MobiControl/api/token";

            string body = $"grant_type=password&username={userName}&password={passwd}"; //"grant_type=password&username=Administrator&password=1";
            var payload = new StringContent(body, Encoding.UTF8, "application/json");

            using (var client = new HttpClient())
            {
                var authHeader = Encoding.ASCII.GetBytes($"{ClientID}:{ClientScrt}");
                client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Basic", Convert.ToBase64String(authHeader));

                var result = client.PostAsync(url, payload);

                var token = await result.Result.Content.ReadAsStringAsync();
                AuthToken authToken = JsonConvert.DeserializeObject<AuthToken>(token);

                Console.WriteLine(token);

                ApiHelper.InitializeClient();

                return authToken;
            }
        }

and here's my Client constructor:

the commented line makes the problem(s), it gives me two faults: "An object reference is required for the nonstatic field, method, or property 'member'"

 public static class ApiHelper
    {
        public static HttpClient ApiClient { get; set; }

        public static void InitializeClient()
        {
            ApiClient = new HttpClient();

            ApiClient.DefaultRequestHeaders.Accept.Clear();
            ApiClient.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));

            //ApiClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue(AuthToken.Token_type, AuthToken.Access_token);

        }
    }