myhub.cs

var myHub = connection.CreateHubProxy("PushNotificationsHub");
            connection.Start().ContinueWith(task =>
            {
                try
                {
                    if (task.IsFaulted)
                    {
                        MessageBox.Show("There was an error during open of the connection: " +
                                        task.Exception.GetBaseException());
                    }
                    else
                    {
myHub.Invoke<string>("Subscribe", "TestNotification").Wait();
                    }
                }
                catch (Exception ex)
                {
                    MessageBox.Show("Error: " + ex.Message.ToString());
                }
            }).Wait();