I've found some code for Map case:
Map myMap = new Map();
String filepath = "location_of_raster_file" ;
Raster myRasterFile = new Raster(filepath);
RasterLayer myRasterLayer = new RasterLayer(myRasterFile);
await myRasterLayer.LoadAsync();
myMap.Basemap.BaseLayers.Add(myRasterLayer);
But it doesn't work for Scene:
Scene newScene = new Scene();
String filepath = "C:\\Users\\user\\Desktop\\City.tpk";
Raster myRasterFile = new Raster(filepath);
RasterLayer myRasterLayer = new RasterLayer(myRasterFile);
await myRasterLayer.LoadAsync();
newScene.Basemap.BaseLayers.Add(myRasterLayer);