We use a channel manager called channex.
Request connection to Channex.io in booking extranet
Login to the admin for the property here: https://account.booking.com/
This step is best done by the property since booking.com have 2 step security with passcodes sent to the phone.
Copy the property code at the top of the navigation, you will need this later inside Channex to connect the account
Click on Account > Connectivity Provider
Choose Provider Screen
Click on "Search"
Type "Channex" and it will find Channex.io on the list.
You have to type the whole word "Channex" since it wont find it otherwise.
Once channex is selected on the list it will show the summary box, just click "Next"
Agree the XML Service Agreement
Click on the checkbox to agree the terms and conditions and then the "Yes, I accept" button.
No other things needs to be done or completed on this form
Now you will be in a waiting status, until Channex accepts the connection