It is pointless to hide the backend and an exercise in futility if you have real security measures in place, but you could use this Simple Query String Login page protection method if you want to do something like that anyway.
Your WordPress Admin backend area is already protected with authentication. In order to get into your /wp-admin folder someone has to be authenticated to your /wp-admin folder / WP Dashboard. In order to cause a problem for your website that person would need to be an Administrator. If you use BPS Pro Login Security and BPS Pro JTC Anti-Spam / Anti-Hacker then your /wp-admin folder and Dashboard area are protected against auto-posting spambots and hackerbots, which make up 99%+ of Brute force attacks, spam registrations, etc.