ArtStation has a little feature for hiding projects so that it does not show on your portfolio, but it can be accessed by people who know the URL.
When creating or editing a project, on the right panel there is a panel 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.