We are really sorry to say that, it’s not possible to hide the main menu & page breadcrumb for a specific page like WHMCS Bridge page. It’s only possible if you want to hide both from your full website.
Moreover, the main menu is to navigate your website and the WHMCS menu (that shows in the WHMCS Bridge page) is to navigate the client area & billing. If you hide the main menu, then how your customer will navigate back to your website from the WHMCS Bridge page, if they are looking another service.
Thanks for your understanding.
Best Regards,
ThemeLooks Support Team