firebaseOnceWithAuth.cs

// authenticate user with facebook and use the access token
var token = "access_token_xyz";

// specify your app’s client key when creating the auth provider
var ap = new FirebaseAuthProvider(new FirebaseAuthConfig("<YOUR APP API KEY>"));

// sign in with OAuth. You can also sign in anonymously
var auth = ap.SignInWithOAuth(FirebaseAuthType.Facebook, token);

var firebase = new FirebaseClient(
  "https://dinosaur-facts.firebaseio.com/",
  new FirebaseOptions
  {
    AuthTokenAsyncFactory = () => auth.FirebaseToken
  });

var dinos = await firebase
  .Child(“dinosaurs”)
  .OnceAsync<Dinosaur>();