sketch-polygon.js

const sketchLayer = new GraphicsLayer({
  elevationInfo: {
    mode: "on-the-ground"
  }
});
webscene.add(sketchLayer);

const sketchViewModel = new SketchViewModel({
  layer: sketchLayer,
  view: view
});

sketchViewModel.on("create", function (event) {
  if (event.state === "complete") {
    console.log(event.graphic.geometry);
    // apply a spatial filter on the buildings
    // using the graphic's geometry
  }
});

document.getElementById("drawPolygon").addEventListener("click", function () {
  sketchViewModel.create("polygon");
});