From the Booking.com website:
Log in to your Booking.com account at admin.booking.com
Go to Rates & Availability and then to Calendar, or click on Calendar & Prices, depending on your extranet version.
Click on Sync Calendars.
Click on Connect another calendar and add the link of the external calendar from which you want to import reservations. Choose a name for the connection (e.g., if the connection is with Airbnb, you can name it "Airbnb") and then click on Next Step. You can also click Skip this step if you want to go straight to exporting the calendar. You will be able to import the external calendar later by clicking Complete Setup on the Sync Calendars page.
To export the Booking.com calendar, click on Copy Link and paste it in the corresponding field of the external calendar, for example, on the Airbnb calendar synchronization page. In Export Options, choose whether to export Only reserved dates or Reserved and closed dates. You can also click on Skip export; you can export your Booking.com calendar at any time by clicking Complete Setup on the Sync Calendars page.
From the Check-in Scan website:
Log in to the web control panel of your account.
Click on the Properties section.
Click on Edit.
Inside the property settings:
Click on the iCal tab.
Simply paste the link extracted from Booking.com and then click on Import iCal.
In a few minutes, your reservations from Booking.com will start syncing in the Reservations tab of Check-in Scan.