If you are an online check-in provider, for questions and requests, please reach out to our Solution Engineering Team via checkin.provider[at]booking.com. If you are a Booking.com partner property, you can reach out to our Partner Support Teams from here
Booking.com Online Check-in APIs to enable the integration between Booking.com and Check-in Providers: https://suiteportal.booking.com/docs/v1.3/