it's right there after the "The Domain Search Feature" heading. Verify ownership, then you get results
I'm not sure I really need it for personal use, more just a cool thing to see, so I'm a bit undecided on paying for the domain feature. I can see it being useful for a business though where each email is a different employee dealing with accounts everywhere.
I wish HIBP had a solution for those of us who are individuals but use domain catchall forwarding as our method for separating accounts.
It feels good to see adobe@mydomain.com, newrelic@mydomain.com, internetarchive@mydomain.com, etc. there but not any of the addresses I use for normal communication.
I wish there was a "I'm just one person, or a small family" tier for this.