Foxit has shifted its model recently, so your "key" might actually be your login credentials:
Used for monthly or yearly subscriptions. Instead of a code, you simply click Sign In and use the email address associated with your purchase. PDF Software Online Store | Foxit foxit phantompdf 10.1 activation key
Many organizations and universities provide Foxit licenses to their members. If you are a student or employee, check your institution's software portal. Foxit has shifted its model recently, so your
Used for perpetual (one-time purchase) licenses. Foxit has shifted its model recently