To protect a directory with a password, perform the following steps:
1 - Logged in to Plesk, https://your domain name:8443.
2 - Go to Websites & Domains.
3 - Click Password-Protected Directories.
4 - Click Add Protected Directory.
5 - Type in the path the directory you want to protect.
6 - Click OK.
https://youtu.be/37KC9dTw5sE