public sealed class FeatureSceneLayer : CompositeLayer, ArcGIS.Desktop.Core.IMetadataInfo, ILayerContainer, IMetadataSource, ITableDefinitionQueries, System.ComponentModel.INotifyPropertyChanged  

var sceneLayerUrl = @"https://myportal.com/server/rest/services/Hosted/SceneLayerServiceName/SceneServer";
//portal items also ok as long as the portal is the current active portal...
//var sceneLayerUrl = @"https://myportal.com/home/item.html?id=123456789abcdef1234567890abcdef0";

await QueuedTask.Run(() =>
{
  //Create with initial visibility set to false. Add to current scene
  var createparams = new LayerCreationParams(new Uri(sceneLayerUrl, UriKind.Absolute))
  {
    IsVisible = false
  };

  //cast to specific type of scene layer being created - in this case FeatureSceneLayer
  var sceneLayer = LayerFactory.Instance.CreateLayer<Layer>(
           createparams, MapView.Active.Map) as FeatureSceneLayer;
  //or...specify the cast directly
  var sceneLayer2 = LayerFactory.Instance.CreateLayer<FeatureSceneLayer>(
           createparams, MapView.Active.Map);
  //ditto for BuildingSceneLayer, PointCloudSceneLayer, IntegratedMeshSceneLayer
  //...
});
      

var featSceneLayer = MapView.Active.Map.GetLayersAsFlattenedList()
                 .OfType<FeatureSceneLayer>().FirstOrDefault();
var scenelayerName = featSceneLayer?.Name;

//var featSceneLayer = ...;
var sname = featSceneLayer.Name;

await QueuedTask.Run(() =>
{
  //Select all features within the current map view
  var sz = MapView.Active.GetViewSize();

  var c_ll = new Coordinate2D(0, 0);
  var c_ur = new Coordinate2D(sz.Width, sz.Height);
  //Use screen coordinates for 3D selection on MapView
  var env = EnvelopeBuilderEx.CreateEnvelope(c_ll, c_ur);

  //HasAssociatedFeatureService does not matter for SelectFeaturesEx
  //or GetFeaturesEx
  var result = MapView.Active.SelectFeaturesEx(env);
  //var result = MapView.Active.GetFeaturesEx(env);

  //The list of object ids from SelectFeaturesEx
  //At 2.x - var oids1 = result.Where(kvp => kvp.Key.Name == sname).First().Value;
  var oids1 = result.ToDictionary().Where(kvp => kvp.Key.Name == sname).First().Value;
  //TODO - use the object ids

  MapView.Active.Map.ClearSelection();
});