This feature is only available on paid plans.
To make your project private, open the project settings and enable the ‘Protect with a password’ option. If you want all your project pages protected, select ‘All pages’ and enter a password. Your visitors will need to enter this password in order to open your project.
If only certain pages should be hidden, choose ‘Selected, click on the previews for all of the pages you want to protect, and set a password.
It’s not possible to set individual passwords for pages within one project.
You can also change the background image shown on the password protection page.