public class CIMPolygonGraphic : CIMShapeGraphic, System.ComponentModel.INotifyPropertyChanged, System.Xml.Serialization.IXmlSerializable  

{
  // Note: must be called on the QueuedTask
  {
    //On the QueuedTask
    //Place a polygon symbol using the mapview extent geometry
    var extent = MapView.Active.Extent;
    //Contract the extent
    var polygonEnv = extent.Expand(-100000, -90000, false);
    //create a polygon using the envelope
    var polygon = PolygonBuilderEx.CreatePolygon(polygonEnv);

    //specify a symbol
    var poly_symbol = SymbolFactory.Instance.ConstructPolygonSymbol(
                          ColorFactory.Instance.GreenRGB);

    //create a CIMGraphic 
    var graphic = new CIMPolygonGraphic()
    {
      Symbol = poly_symbol.MakeSymbolReference(),
      Polygon = polygon,
    };
    graphicsLayer.AddElement(graphic);
  }
}