Hi Oberdan
OK then the solution you probably want to implement is your own Login and Sign up page.
If you use the login and register functions instead of the promptLogin function.
This way you can build a lightbox that loads a page similar to the normal wix one. You can call login() if they are registered or you can call register(). Make sure you have the settings set to the way you need them to be.
If you need more help let me know.
Steve