TL;DR: Make domain-level notification (1) more obvious to find and (2) more salient in the registration form.

Feature not advertised in top bar labels

• "Home" promises e-mail one-time search,
• "Notify me" promises e-mail notification, not registration
• "Domain search" promises, well, one-time domain search.

I suggest you add either a separate label on top bar with a separate form. Or change existing "Domain search" into "Domain search+notification" (yes, it's longer, so see other suggestion below).

Feature not recognizable when found

On https://haveibeenpwned.com/DomainSearch there is only one salient title "Domain search / Search for pwned accounts across an entire domain". No title about "notification".

I suggest you, on https://haveibeenpwned.com/DomainSearch form:

• to add between the "Domain name" entry box and the "Would you like..." another sub-title (h1, h2, whatever) reading "Notify me domain-wide" or similar.
• to change the checkbox label to at least include the word "notification" or "notify", like "Subscribe me for domain-wide notification".

This would make the feature easier to find, which will improve your interesting service.

Thank you.