Where do I get the CD key for this game?

[freudxxx]

When I start the game for the first time, it asked for a CD key. I couldn't find it. Where do I find the CD key, so that I can start this game?

[ShinDigPig]

On you library page for the game, on the right side where it says "forums" and "store page" there should be one called "CD Key"