public class CIMLayerElevationSurface : CIMObject, System.ComponentModel.INotifyPropertyChanged, System.Xml.Serialization.IXmlSerializable  

//Define the custom elevation surface to use
//At 2.x - var layerElevationSurface = new CIMLayerElevationSurface
//{
//  MapElevationID = "{3DEC3CC5-7C69-4132-A700-DCD5BDED14D6}"
//};
var layerElevationSurface = new CIMLayerElevationSurface
{
  ElevationSurfaceLayerURI = "https://elevation3d.arcgis.com/arcgis/services/WorldElevation3D/Terrain3D/ImageServer"
};
//Get the layer's definition
var lyrDefn = featureLayer.GetDefinition() as CIMBasicFeatureLayer;
//Set the layer's elevation surface
lyrDefn.LayerElevation = layerElevationSurface;
//Set the layer's definition
featureLayer.SetDefinition(lyrDefn);