Well, First then nothing, I don't speak english very well..
My problem is the next, I'm trying to implement a Google Login in my app, with next two references:
1. https://jorgediegocrespo.wordpress.com/2018/12/19/login-en-google-desde-xamarin-forms/
2. timothelariviere.com/2017/09/01/authenticate-users-through-google-with-xamarin-auth/
Well, when I select a google account the next page that appear "google.com", and I dontn't knows why.
First: OPEN THE NATIVE BROSER
Seccond: com.companyname.Auth, is my package android name in the AndroidManifest.xml
Next, When I select an account appear this page with a label "Espera un momento ~ Wait a moment"
And for last appear the Google homepage and don't return to my App 
When I close the Navigation native Browser appear this Message
In Google Developer Console my OAth Consent Screem is this:
When I call the OAth2 Constructor the redirect URL than i use is this:
com.companyname.Auth:/oauth2redirec
My Source is the same than the first url reference:
I used in one moment the video tutorial of Houssem Dellai...
https://youtube.com/watch?v=AgFIsVr26zg
In his tutorial he uses a webview in the shared code, but i readed that google don't admit this solution now, and when i run the webview i have another error from Google.
I don't understand what is my problem.. i have several days trying this please help!!