How to get an access token with the OAuth 2.0 in the Unity Android Application?

I have the following scenario that needs to be implemented for our game: we use GameSparks and silent device authentication and if our player wants to get his data on the other devices, we give him an opportunity to register a new account using username and password or one of the services. These services are: Google Play and Game Center. But the problem is that I need to provide an access token for using GooglePlayConnectRequest, and I can't get this access token using Android Native Plugin, it just returns an empty string where the access token is supposed to be. Have anybody been able to get the access token, if yes - could you provide some tutorial or instruction? Maybe I need to use another plugin which gets the access token without any problems?

Thanks in advance!