public class PopupDefinition 

{
  if (mapView == null) return;
  // Sample code: https://github.com/ArcGIS/arcgis-pro-sdk-community-samples/blob/master/Map-Exploration/CustomIdentify/CustomIdentify.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.ShowPopup(mapMember, objectID, popupDef);
}

{
  if (mapView == null) return;

  //Create custom popup content
  List<PopupContent> popups =
  [
      new("<b>This text is bold.</b>", "Custom tooltip from HTML string"),
          new(new Uri("https://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.ShowCustomPopup(popups, null, true, popupDef);
}