WM 7, C# await, async return void

public void GetLastSeenCompleted(string did)
       {
           houdiniMMSService.GetLastSeenCompleted += new EventHandler<HoudiniMMSService.GetLastSeenCompletedEventArgs>(HoudiniMMSServiceGetLastSeenCompleted);
           houdiniMMSService.GetLastSeenAsync(did);

       }


       private void HoudiniMMSServiceGetLastSeenCompleted(object sender, HoudiniMMSService.GetLastSeenCompletedEventArgs e)
       {
           try
           {
               //I want to stop executing my program until this message is displayed
               // How can I do this?

               MessageBox.Show(Convert.ToString(e.Result));
           }
           catch(Exception ex)
           {
               MessageBox.Show("Unable to get last seen value from server: " + ex.ToString());
           }
       }