protected CIMSymbolReference SketchSymbol {get; set;}

internal class SketchTool_WithSymbol : MapTool
{
  public SketchTool_WithSymbol()
  {
    IsSketchTool = true;
    SketchOutputMode = SketchOutputMode.Map; //Changing the Sketch Symbol is only supported with map sketches.
    SketchType = SketchGeometryType.Rectangle;
  }

  protected override Task OnToolActivateAsync(bool hasMapViewChanged)
  {
    return QueuedTask.Run(() =>
    {
      //Set the Sketch Symbol if it hasn't already been set.
      if (SketchSymbol != null)
        return;
      var polygonSymbol = SymbolFactory.Instance.ConstructPolygonSymbol(ColorFactory.Instance.CreateRGBColor(24, 69, 59),
                          SimpleFillStyle.Solid,
                        SymbolFactory.Instance.ConstructStroke(ColorFactory.Instance.BlackRGB, 1.0, SimpleLineStyle.Dash));
      SketchSymbol = polygonSymbol.MakeSymbolReference();
    });
  }
}