I don't see why you wouldn't have a small login button somewhere for admins to access a login page. Anyway, some options:
- create a login page with a dedicated url so admins can manually navigate and login via e.g. {yoururl}/p/login while anymous users don't see this option in the menu or on their pages
- if you really don't want admins to login, create an endpoint and fetch/update the necessary data from another app where admins do login
A login system doesn't mean everyone needs to pass a login page. You can have a homepage and navigation for anonymous users and put a login page behind a button / link / menu item for other user roles.
In Mendix I'd solve this with the login.html page that is available in your application's directory. Implement the administration module for managing the admin users and then the admin user would be able to login by navigating to your app's URL followed by '/login.html'