Get Server Auth Code for LoginWithGoogleAccountRequest in C# and PlayFab API

Question

I am using PlayFab for creating accounts in my Desktop Unity game. I want to allow users to login using google in my game. However, I do not know how to get the Server Auth code that LoginWithGoogleAccountRequest needs:

My code looks something like this:

using PlayFab;
using PlayFab.ClientModels;
using UnityEngine;
 
 
public class LoginManager: MonoBehaviour
{
    string GetServerAuthCode() {
 
    }
 
    void LoginUsingGoogle() {
        var request = new LoginWithGoogleAccountRequest { CreateAccount = true, ServerAuthCode = GetServerAuthCode()};
        PlayFabClientAPI.LoginWithGoogleAccount(request, OnLoginUsingThirdPartySuccess, OnLoginUsingThirdPartyFailure);
    }
}

I want to write the appropriate code under GetServerAuthCode.

I know how to get the token, but I do not know how to get the server auth code:

 public string GetToken() {
        string[] scopes = {"https://www.googleapis.com/auth/userinfo.email", "https://www.googleapis.com/auth/userinfo.profile"};
        var credentials = GoogleWebAuthorizationBroker.AuthorizeAsync(
            new ClientSecrets {
                ClientId = ClientId,
                ClientSecret = SecretKey,
            }, scopes, "user", CancellationToken.None);
        return credentials.Result.Token.AccessToken;

    }