Allow existing users to link their Google account?

I’ve set up Google OAuth to let users sign up and in using their Google account.
I let these users set up a password, so that they can either use their Google account or their email-password combination to sign in.

For those who did not sign up using their Google account, I’d like to let them link it, so that they’d have the same options as other users.

I’m a bit confused as to how to do this, since the only inputs I’ve seen for Google OAuth in Xano are code and redirect_uri.
Where do I retrieve the code value from?