Invisible client pages with password protection

Please can you confirm how I should create scheduled password-protected posts which clients can see? I’ve tried sending links to these pages but anyone not logged into the WP admin gets a ‘Page not found’ message. I’ve worked around it by creatinng subscriber user accounts but I didn’t think this would be necessary. I want to be sure the pages are entirely invisible while they’re scheduled, whilst also being able to share them with specific people.

Many thanks,
Lindsey