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

{
    //Note: needs to be called on the MCT
    //Define the custom elevation surface to use
    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);
}