*Set up an Auto Action (SETTINGS-GUEST MANAGEMENT-AUTO ACTIONS) which selects the bookings which will be sent to Mailchimp. Set "Trigger Action" = automatic.
*Select the Auto Action which you want to use for Mailchimp.
*Go to SETTINGS-> APPS & INTEGRATION -> MAILCHIMP)
*Enter the Mailchimp API key
*Enter the Mailchimp list ID (without the . at the end)