On a Pro site:
• Go to "Site Settings" and select "Password Protection"
• Enable password protection with the switch on the right, and hit Apply.
• A Shield icon will appear at the top of your pages.
• Go to the page you want to protect with a password, and tap the shield icon.
• Insert a user name and password for that page.
• Publish your site for changes to take effect.
Watch the tutorial video:
How To Protect A Page With A Password