public Geometry Geometry {get;}

var tinLayer = MapView.Active.Map.GetLayersAsFlattenedList().OfType<TinLayer>().FirstOrDefault();
await QueuedTask.Run(() =>
{
  if (tinLayer.CanGetZs())
  {
    // get z value for a mapPoint
    var zResult = tinLayer.GetZs(mapPoint);
    if (zResult.Status == SurfaceZsResultStatus.Ok)
    {
      // cast to a mapPoint
      var mapPointZ = zResult.Geometry as MapPoint;
      var z = mapPointZ.Z;
    }

    // get z values for a polyline
    zResult = tinLayer.GetZs(polyline);
    if (zResult.Status == SurfaceZsResultStatus.Ok)
    {
      // cast to a Polyline
      var polylineZ = zResult.Geometry as Polyline;
    }
  }
});