AWS Contigo - authenticate to Google on my Flutter app

I'm trying to implement Login Screen with Flutter that will authenticate with Google throw AWS Contigo authentication.

I'm using this library with Cognito's federated sign-in on mobile devices. I'm using flutter_webview to navigate to Cognito's authorized URL. Use flutter_webview's navigationDelegate to catch the redirect from the Google app. then I should make a POST request to Cognito's token URL to get your tokens. Create the session and user with the tokens.

I'm not sure what I should add in the redirect to Google?

https://${COGNITO_POOL_URL}" +
        ".amazoncognito.com/oauth2/token?grant_type=authorization_code&client_id=" +
        "${COGNITO_CLIENT_ID}&code=" + authCode + "&redirect_uri=myapp://WHAT TO ADD HERE??";

Thank you.