Password protection will require a User Name and Password in order to lock your Public Link or Custom domain behind a password.
This protection will apply to the entire Prototype or Website.
It is currently not possible to only lock one single screen or page.
How to Add Password Protection
Go to the Project you want to password protect
Click on the Share button in the top right
Go to the Prototype Link tab
Enable the Prototype link
Go to the Password Protection section, and enable
Copy and share your credentials