This feature is only available with a Premium Account.
As a Premium user, you are able to hide a project so that it won't be visible on your portfolio or website, but people who know the URL can access it.
When creating or editing a project, on the right sidebar there is a block for "Visibility" with 2 checkboxes to show the project on ArtStation.com or your website. Uncheck them both and save the project. Ensure your project is Published.
When in this mode, the project is "unlisted". It won't show up anywhere but is still visible by people who have the URL.
To get the URL, click on "View on Community" and retrieve the URL from the browser.