@JeremyCherfas A couple tricks come to mind. No clue if anything Wordpress natively supports either.
One is to have a hard-to-guess URL. Think like a UUID. That shows the secret post when visited.
The other is to encode auth info in the URL you share via email. Email havers auto-auth. Anyone else doesn’t know username and password.
Throw in time-limiting for extra fun. And maybe per-email-subscriber URLs so you can tell who shares the secret with others.