Make domain notification more salient
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.
Good point, I missed it too.
Note that when we verify a domain, which I did via a txt record, we are then told we can delete the record. What happens if the domain changes hands? I'd suggest the txt record should stay and that it should be checked before new notifications.