Your license is your forwarding destination, which is either an email address or a phone number.
After you pay, your forwarding destination is added to the PhoneLeash license database. Often this is not the same email you are writing from, so right after paying the invoice, please specify where you are forwarding to.
After your forwarding destination is licensed, the app status notification will say "Active [Licensed]"
When you change phones, if you forward to the same destination, your license will automatically become active on the new phone.