That key is unique to each game, unlike the one UBI published for single player. There are two, one is usually on the CD case and the other in or on the back of the manual.
If you don't have it, you may be out of luck. Try contacting UBI support to see if they can help, but I don't think they 'support' Chaos Theory anymore.
Only other solution would be to find a copy that HAS the online key and get it as the game is so cheap now.