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

public void ShowCustomPopup(MapMember mapMember, long objectID)
{
  //Get the active map view.
  var mapView = MapView.Active;
  if (mapView == null)
    return;

  //Create custom popup content from existing map member and object id
  var popups = new List<PopupContent>();
  popups.Add(new PopupContent(mapMember, objectID));
  
  //Create a new custom command to add to the popup window
  var commands = new List<PopupCommand>();
  commands.Add(new PopupCommand(
    p => MessageBox.Show(string.Format("Map Member: {0}, ID: {1}", p.MapMember, p.ID)),
    p => { return p != null; },
    "My custom command",
    new BitmapImage(new Uri("pack://application:,,,/ArcGIS.Desktop.Resources;component/Images/GenericCheckMark16.png")) as ImageSource));

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