You ID number consists of a few digits. It is visible in the numbers of your reservations done by a specific booking portal. It's  the digits at the very beginning, which are always the same. For example: 12345 xxxxxxxxxxx
You can also check your id by entering the site of your hostel on a specific booking portal. Then check the link in your internet browser. It shoul look like that(for HostelWorld): of the Hostel/Location of the hostel/12345 <--- the nuber at the end is your hostel's ID number.

It is necessary to know the ID number of your hostel before establishing the XML connection with the booking portals.