I was following this tutorial for .Net and UWP.
Author, edit, and save maps to your portal—ArcGIS Runtime SDK for .NET | ArcGIS for Developers
There I connect to my ArcGis Online account using my user/password. I create a Feature Collection local add features and save the map + feature collection on my content in my Online account.
But what I need is get one layer already on my ArcGis Online add more features and save it.