Trying to get the sample to work for WmtsService - arcgis-runtime-samples-dotnet/WMTSLayer.xaml.cs at master · Esri/arcgis-runtime-samples-dotnet · GitHub
// Define the Uri to the WMTS service.
Uri wmtsUri = new Uri("https://sampleserver6.arcgisonline.com/arcgis/rest/services/WorldTimeZones/MapServer/WMTS");
WmtsService myWmtsService = new WmtsService(wmtsUri);
await myWmtsService.LoadAsync();
the LoadAsync() throws and exception

I've tried various formats of the URL including ?request=GetCapabilities&service=WMTS and a different service but get the same error. Please advise.
Thanks,