This function will allow you to use your own domain e.g. bookings.mydomain.com (instead of beds24.com) and will remove the Beds24 link from the booking page.
This function will also allow you to use the your own domain in the payment link https://beds24.com/bookpay.php?bookid=11111111 to https://mydomain.com/bookpay.php?bookid=11111111
The domain must be unique and cannot be exactly the same domain as your website, usually creating a sub domain for the booking page is a good idea.