_mappackage = await OpenAsync("D:\\Source code\\Data\\Test.mmpk");
var mapPackageUri = new Uri(_mappackage.Item.Url);
var mapTable = new ServiceFeatureTable(mapPackageUri);
var mapPackageFeatureLayer = new FeatureLayer(mapTable);
myMap.OperationalLayers.Add(mapPackageFeatureLayer);
More specifically I'm not sure what to do for the OpenAsync method.