Making calls to the HIBP email address search API requires a subscription which gives you a rate limited key. The same key allows you to query domains you've successfully demonstrated control of via the domain search feature, as well as search those domains via the web interface. If you've already purchased a subscription, you'll be able to manage it after verifying you have access to the email address you wish to use (you'll receive a unique link to that address). If you need help, check out the FAQs and troubleshooting guide or submit a support request.
† Only domains that have been added to the domain search dashboard after successfully verifying control can be searched. The number of breached accounts excludes any that appear solely in breaches flagged as spam lists.