When contacting your provider for support, in general there are two important changes you'll want to enquire about:
First, that your A record is pointing to the Booking ChoicePro IP address 126.96.36.199
And second, that the 'www' CNAME record for your site redirects to suite-sites.booking.com
You can use the template below to contact your provider and request these changes be made:
I am reaching out to connect my domain (INSERT YOUR DOMAIN HERE), which I registered with your service for my website. To set this up correctly, I need to change the A record for my root domain to point to 188.8.131.52. In addition, I would like to create a redirect for my 'www' CNAME record to 'suite-sites.booking.com'.
Note that I am not requesting a transfer of my domain – I will continue to manage it with your service. Please make the required changes to my account.