NetworkSystemLayers

const [WebMap, MapView, esriConfig] = await $arcgis.import([
  "@arcgis/core/WebMap.js",
  "@arcgis/core/views/MapView.js",
  "@arcgis/core/config.js"
]);
let utilityNetwork;

// set the hostname to the portal instance
esriConfig.portalUrl = "https://myHostName.domain.com/arcgis";

const webMap = new WebMap({
  portalItem: {
    id: "webmapID"
  }
});

const mapView = new MapView({
  map: webMap
});

webMap.when(async () => {
  // check if webMap contains utility networks
  if (webMap.utilityNetworks.length > 0) {
    // assign the utility network at index 0
    utilityNetwork = webMap.utilityNetworks.at(0);

    // trigger the loading of the UtilityNetwork instance
    await utilityNetwork.load();

    // Print the subnetworks table service url and id
    console.log(`Dirty areas layer id: ${utilityNetwork.networkSystemLayers.subnetworksTableId}`);
    console.log(`Dirty areas layer url: ${utilityNetwork.networkSystemLayers.subnetworksTableUrl}`);
  }
});