public sealed class PopupCommand : ArcGIS.Desktop.Framework.Contracts.PropertyChangedBase, System.ComponentModel.INotifyPropertyChanged  

{
  //Create custom popup content from existing map member and object id
  List<PopupContent> popups = [new PopupContent(mapMember, objectID)];

  //Create a new custom command to add to the popup window
  List<PopupCommand> commands =
  [
    new PopupCommand(
          p => MessageBox.Show(string.Format("Map Member: {0}, ID: {1}", p.MapMember, p.IDString)),
          p => { return p != null; },
          "My custom command",
          System.Windows.Application.Current.Resources["GenericCheckMark16"] as ImageSource),
      ];

  mapView.ShowCustomPopup(popups, commands, true);
}