You can enable domain level authentication for your website. This authentication is at the server / domain level and will prompt visitors to enter a username and password before they can proceed further.
The benefits of this level of authentication is typically for websites / apps that are in development and where you want only authorized users to be able to view.
To enable, go to the server that houses the sites you want to add authentication for > select the site > select settings > and then select Lock Site. Then, add users (username and password) that can access the site.
You can add new users and delete existing users.
To disable authentication, select option to Unlock Site.