public class ElevationLayerCreationParams : LayerCreationParams 

//Note: call within QueuedTask.Run()
//Define a ServiceConnection to use for the new Elevation surface
var serverConnection = new CIMInternetServerConnection
{
  Anonymous = true,
  HideUserProperty = true,
  URL = "https://elevation.arcgis.com/arcgis/services"
};
CIMAGSServiceConnection serviceConnection = new CIMAGSServiceConnection
{
  ObjectName = "WorldElevation/Terrain",
  ObjectType = "ImageServer",
  URL = "https://elevation.arcgis.com/arcgis/services/WorldElevation/Terrain/ImageServer",
  ServerConnection = serverConnection
};
//Defines a new elevation source set to the CIMAGSServiceConnection defined above
//At 2.x - var newElevationSource = new ArcGIS.Core.CIM.CIMElevationSource
//{
//  VerticalUnit = ArcGIS.Core.Geometry.LinearUnit.Meters,
//  DataConnection = serviceConnection,
//  Name = "WorldElevation/Terrain",
//  Visibility = true
//};
//The elevation surface
//At 2.x - var newElevationSurface = new ArcGIS.Core.CIM.CIMMapElevationSurface
//{
//  Name = "New Elevation Surface",
//  BaseSources = new ArcGIS.Core.CIM.CIMElevationSource[1] { newElevationSource },
//  Visibility = true,
//  ElevationMode = ElevationMode.CustomSurface,
//  VerticalExaggeration = 1,
//  EnableSurfaceShading = false,
//  SurfaceTINShadingMode = SurfaceTINShadingMode.Smooth,
//  Expanded = false,
//  MapElevationID = "{3DEC3CC5-7C69-4132-A700-DCD5BDED14D6}"
//};
//Get the active map
var map = MapView.Active.Map;
//Get the elevation surfaces defined in the map
//At 2.x - var listOfElevationSurfaces = definition.ElevationSurfaces.ToList();
var listOfElevationSurfaces = map.GetElevationSurfaceLayers();
//Add the new elevation surface 
//At 2.x - listOfElevationSurfaces.Add(newElevationSurface);
var elevationLyrCreationParams = new ElevationLayerCreationParams(serviceConnection);
var elevationSurface = LayerFactory.Instance.CreateLayer<ElevationSurfaceLayer>(
       elevationLyrCreationParams, map);

// wrap in QueuedTask.Run

// surfaceLayer could also be the ground layer

string uri = "https://elevation3d.arcgis.com/arcgis/rest/services/WorldElevation3D/Terrain3D/ImageServer";
var createParams = new ElevationLayerCreationParams(new Uri(uri));
createParams.Name = "Terrain 3D";
var eleSourceLayer = LayerFactory.Instance.CreateLayer<Layer>(createParams, surfaceLayer);