First, start up ET, open the console with the tilde (~) key, and type in /pb_myguid
Write down the last 8 characters of the GUID string you see on the right (not the CD Key string).
Exit out, and come back to the TWL home page. If you're logged in, you'll see your name on the left hand side of the menu bar. Mouse over it, and go to Account Maintenance -> Profile. Click on Profile.
In the box labeled "Anti-Smurf In Game Identifiers" you'll see a link entitled "add another id". Click on it. On the next page, select Enemy Territory GUID in the ID Type and type in the 8 characters you wrote down. Click on Add Identifier, and you're done.
You can use pb_plist or pb_ power in the server to display the guids.
You can check them by clicking on the players name on the website or searching in "Find Player By In Game Identifier" in your personal menu. Again, you should only need the last 8 digits.