How to password protect multiple WP-Admin folders


I have a dedicated server with over 100 WP sites and the server is under brutal attack for a few days.

Is it possible to password-protect (directory confidentiality) the Wp-admin folder for all sites via command line / ssh so that it can display a contextual connection before the actual login page (example: picture-copy-2)

I remember my old host company doing it via a command line. my current host says that it is not possible.