Making calls to the HIBP API requires a key. There's a full blog post on why here, this page allows you to either purchase one for a single month, on a recurring subscription charged monthly or manage an existing subscription (i.e. cancel it). There's a US$3.50 per month fee, the reasons for which are explained in the aforementioned blog post. If you've already purchased a key, 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). Common questions relating to API keys are addressed on the FAQs page.