Our websites follow a very simple structure and password-protected areas are no feature that we provide. People usually put a link to an email on their site and send confidential files directly to the people who request them.
What you can do, is to create a hidden project that is only accessible through the url:
1. Create a new page and add it to a navigation list.
2. Click on the navigation list, choose "options" and deselect the page (see attachment).
3. You need to create a link to this hidden page somewhere on your site, otherwise the browser won't be able to "find" and display it.
You can still see the page in your content overview and use the url to send it to your client.