public Geometry Sketch {get;}

// BeforeSketchCompleted event is fired by 
//  - COTS construction tools (except annotation, dimension geometry types), 
//  - Edit Vertices, Reshape, Align Features
//  - 3rd party tools with FireSketchEvents = true


//Subscribe to the before sketch completed event
//ArcGIS.Desktop.Mapping.Events.BeforeSketchCompletedEvent.Subscribe(OnBeforeSketchCompleted);

private Task OnBeforeSketchCompleted(BeforeSketchCompletedEventArgs args)
{
  //assign sketch Z values from default surface and set the sketch geometry
  var modifiedSketch = args.MapView.Map.GetZsFromSurfaceAsync(args.Sketch).Result;
  args.SetSketchGeometry(modifiedSketch.Geometry);
  return Task.CompletedTask;
}