public void ShowCustomPopup( 
   IEnumerable<PopupContent> popupContent,
   IEnumerable<PopupCommand> commands,
   bool includeDefaultCommands,
   PopupDefinition popupDef
)

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.IDString)),
    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);
}

public void ShowCustomPopupWithWindowDef()
{
  if (MapView.Active == null) return;

  //Create custom popup content
  var popups = new List<PopupContent>
        {
            new PopupContent("<b>This text is bold.</b>", "Custom tooltip from HTML string"),
            new PopupContent(new Uri("http://www.esri.com/"), "Custom tooltip from Uri")
        };
  // Sample code: https://github.com/ArcGIS/arcgis-pro-sdk-community-samples/blob/master/Framework/DynamicMenu/DynamicFeatureSelectionMenu.cs
  var topLeftCornerPoint = new System.Windows.Point(200, 200);
  var popupDef = new PopupDefinition()
  {
    Append = true,      // if true new record is appended to existing (if any)
    Dockable = true,    // if true popup is dockable - if false Append is not applicable
    Position = topLeftCornerPoint,  // Position of top left corner of the popup (in pixels)
    Size = new System.Windows.Size(200, 400)    // size of the popup (in pixels)
  };
  MapView.Active.ShowCustomPopup(popups, null, true, popupDef);
}