public class ElevationLayerCreationParams : LayerCreationParams 

{
    //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
    //Get the elevation surfaces defined in the map
    var listOfElevationSurfaces = map.GetElevationSurfaceLayers();
    //Add the new elevation surface 
    var elevationLyrCreationParams = new ElevationLayerCreationParams(serviceConnection);
    //Note: needs to be called on the MCT
    var elevationSurface = LayerFactory.Instance.CreateLayer<ElevationSurfaceLayer>(
        elevationLyrCreationParams, map);
}

{
    ElevationSurfaceLayer surfaceLayer = null;
    // 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";
    //Note: needs to be called on the MCT
    var eleSourceLayer = LayerFactory.Instance.CreateLayer<Layer>(createParams, surfaceLayer);
}