apiclient.cs

static void Main(string[] args)
        {
            HttpClient client = new HttpClient();

            // STS
            string cloud = "https://login.microsoftonline.com";
            string tenantId = "e0e04c7d-4d23-4078-9562-cea9be7bffed";
            string authority = $"{cloud}/{tenantId}";

            // Application
            string clientId = "ed97e261-0f45-4123-33921-e5f6119da09c";
            string appKey = "eR9klLZqNJwTzew4NYeEL/nfVv8HaqOP67MrFRpihwB=";
            Uri redirectUri = new Uri("http://localhost");

            // Application ID of the Resource (could also be the Resource URI)
            string resource = "b4439d32-e7b0-124d-9858-a18ea2ccee99";
            var clientCredential = new ClientCredential(clientId, appKey);

            AuthenticationContext ac = new AuthenticationContext(authority);
            AuthenticationResult result = ac.AcquireTokenAsync(resource, clientCredential).Result;

            client.BaseAddress = new Uri("https://kloud.azure-api.net");
            client.DefaultRequestHeaders.Accept.Clear();
            client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));

            client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", result.AccessToken);

            try
            {
                var apiResult = client.GetStringAsync("/simpleecho/echo").Result;
                Console.WriteLine(apiResult);
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex);
            }
            

            Console.ReadLine();
        }