Add a return key value pair to an existing GoogleSignInOptions (gso)

The key-value is a client state that is processed this way:

Server -> App -> GSO -> App -> Server

This looks like a very simple question, but I cant seem to find a solution to this problem after much googling. The documentation on Builder did not reveal much as well. But I cant seem to trust my self that the Devs missed this very basic task.

Initializing gso looks like this:

gso = new GoogleSignInOptions.Builder(GoogleSignInOptions.DEFAULT_SIGN_IN)
    .requestEmail()
    .build();
mGoogleSignInClient = GoogleSignIn.getClient(this, gso);

Then user event sign in launches the intent through this function:

private void signIn(key, value) {

    Intent signInIntent = mGoogleSignInClient.getSignInIntent();
    startActivityForResult(signInIntent, RC_SIGN_G);

}

The signIn function possess the key-value pair, but I am stuck how to proceed further.

I also could not find a documentation to tell gso to return the key value pair after user sign in.