protected string SketchTip {get; set;}

public class SketchToolWithSketchTip : MapTool
{
  public SketchToolWithSketchTip()
  {
    IsSketchTool = true;
    SketchType = SketchGeometryType.Line;
    SketchOutputMode = SketchOutputMode.Map;
    SketchTip = "hello World";
  }
}