Store Data in Firebase Database

Hi, I'm trying to store User's data in the Firebase Database. I add to the project this API https://github.com/rlamasb/Firebase.Xamarin .
There is a better options? I can't use .WithAuth parameter and i don't know why. This is my code:

 var firebase = new FirebaseClient(URLDATABASE);
                            var item = await firebase
                                .Child(string.Format("USER_TABLE/{0}/",user.ID))
                                .PostAsync(user);

I need store a class into the Database if possible, so if you can suggest me a similar option. Now i can't use database because i need pass the auth token.