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

//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);