You have to put it in the 'cargo id' field on their track and trace page (which can be found under "customer centre"
From what I read, it will only display your car there if they actually have taken possession of the car and it might have a delay. Some people never have their car showing up there apparently while they still ship with WW but with a different operator.
My VIN doesn't show anything yet, unfortunately.. they get 3 more days (since it's the 22nd in Germany in about 30 minutes and it has to ship out on the 25th) to load my car, or else I will have to wait 2 weeks extra (next ship, if shipped with WW, is going March 10, and then I'm looking more to a late April, perhaps early May delivery... too scary this).
edit: this link might help too: https://att.2wglobal.com/gstattweb/h..._Ocean_GST.htm
edit2: that link did help me a little bit. When you use wildcards, such as the help page says you can do, you can see a LOT more cars and their statuses. It gave me the indication that they're not loading my ship yet, so that's probably why it's not showing up yet. Just do something like this in the cargo ID: WBA3A5C57C% (the first part of the vin is for many cars the same, the percentage makes it search for everything that starts like this. If your vin has a different start, then you can change it of course).