It's important to clarify that is not an official Valve product. It is a third-party modification (mod) of the original Counter-Strike 1.6 , often distributed as a standalone installer. As such, it does not use or require a standard Steam CD key .
If you have a copy of this mod that is asking for a key, here is the most common solution:
CS-CARBON-V1.1-UNLOCK or simply any combination of numbers/letters (e.g., 11111-11111-11111 ).