Where to find CD Key requested on starting a game through Steam?

Some games require an additional key to the one used initially to add a game to your Steam library and then install the game.

Typically such a CD key will be requested as you first start the game after installation and when one is required it is usually displayed as you click to "PLAY" the game from Steam.

If you're asked for a CD Key and one wasn't displayed when you clicked "PLAY" you can:

1. Select the game in your Steam Game library:
   
2. Look for and click on the "CD Key" entry in the "LINKS" section on the right:
   
3. A dialog should appear on which the CD Key will be shown in the "PRODUCT CODE" column:
   
4. Click the "COPY KEY TO CLIPBOARD" button to copy the key so you can then paste it into the dialog shown when you start the game and avoid typing errors:
   

Alternatively, right click on the game in your Steam Library and choose "View CD Key".