protected internal virtual Task<string> OnCreateHtmlContent()

public void ShowDynamicPopup(MapMember mapMember, List<long> objectIDs)
{
  //Get the active map view.
  var mapView = MapView.Active;
  if (mapView == null)
    return;

  //Create popup whose content is created the first time the item is requested.
  var popups = new List<PopupContent>();
  foreach (var id in objectIDs)
  {
    popups.Add(new DynamicPopupContent(mapMember, id));
  }
  
  mapView.ShowCustomPopup(popups);
}

internal class DynamicPopupContent : PopupContent
{
  public DynamicPopupContent(MapMember mapMember, long objectID)
  {
    MapMember = mapMember;
    ID = objectID;
    IsDynamicContent = true;
  }
  
  //Called when the pop-up is loaded in the window.
  protected override Task<string> OnCreateHtmlContent()
  {
    return QueuedTask.Run(() => string.Format("<b>Map Member: {0}, ID: {1}</b>", MapMember, ID));
  }
}