Recover your API key
Your key is computed from your Stripe subscription rather than stored, so it can be reissued at any time. Quote a reference from your receipt and you will get the same key back.
Where to find the reference
Stripe emails a receipt for every payment. It links to a hosted invoice page, and both the invoice and the confirmation carry the subscription id, which starts sub_. If you still have the browser tab from checkout, the session_id in its address is a checkout session id, which starts cs_. Either works.
Why not just an email address?
Because PermitBird has no accounts, an email address cannot be verified here. Accepting one would mean handing out an API key to anybody who could guess where a subscriber works. A Stripe reference is only ever sent to the person who paid.
If the subscription has lapsed
No key is issued for a cancelled or unpaid subscription, because it would be refused on its first call. Start again from pricing, or manage an active subscription at billing.