Your event signing secret, not an API key.
Unchanged request body, read before JSON parsing.
Complete value of the SIGNATURE_HEADER header.
EventSignatureError If the secret is missing, the signature is invalid, or the timestamp is more than five minutes before or after the receiver's clock.
Verify the signature and signing timestamp without parsing the event or running callbacks. Use before storing a delivery in a trusted queue; workers can later use client.parseEventNotificationWithoutVerification() on the stored payload.