var clientHandler = new HawkClientMessageHandler(new HttpClientHandler(), credential, "some-app-data");
var client = new HttpClient(clientHandler);
var request = new HttpRequestMessage(HttpMethod.Get, "http://localhost:8091/api/HelloWorld");
request.Headers.Host = "localhost";
var response = client.SendAsync(request).Result;
string message = response.Content.ReadAsStringAsync().Result;
Console.WriteLine("Response {0} - Http Status Code {1}", message, response.StatusCode);
Additional:
I change url to http://localhost:28290/api/HelloWorld and run Example.Web project,is work well.