The contact.opted_in event is delivered whenever a contact opts back in to receiving messages by sending a keyword message (START, YES, UNSTOP).
Bearer authentication header of the form Bearer <token>, where <token> is your auth token.
The ID of the account in which this event occurred
The data associated with the event
The timestamp when this event occurred, in ISO8601 format
The type of the event. Always contact.opted_in for this event.
"contact.opted_in"Return a 200 status to indicate that the data was received successfully