Log into your Control Panel and click "Websites" at the left side of the page.
Click on "Web settings" under the Website you want to protect with a password and go to "Password protection". Now click on the red slider "Password protect this directory" to activate the password protection.
You can add a name for this protection in the field "Authorization realm:" or leave it empty.
Assign a username and a password. Then confirm it with "Add User" and the user will appear underneath in the list. Click on Save and the Protection will be active.
For removing the protection just click on the now green slider and it changes to red again. Click on Save then the protection will be removed.