A hidden job opening will not be displayed in your openings list and will only be accessible via a direct link. To make an opening hidden, go to the Openings tab of the main navigation bar. Click on the appropriate opening in your openings list. Check the "hidden" box in the Edit Opening form:
Once you have saved the opening, you can copy the direct link to the opening by clicking the arrow icon next to the opening in your openings list. A window will open containing the link.