Where do I use the CD key?

[牛乳を注ぐ女]

This is the CD key of the game. You need to enter the "After application launch" key.

Where does it mean to input the key after launching the game?

[牛乳を注ぐ女]

After this writing, I examined it for a while.
Is it the key to create an account somewhere to get a beta version?
Normally do not need to get it?

[kavika]

It is used for access the Secret Preorder Forum. Instructions are here:
http://forums.wolfire.com/viewtopic.php?f=2&t=16422

On the Secret Preorder Forum, you can talk with other people in the core community (or simply lurk their posts, dating back years).

You can get access to the SUMLauncher there, which has tons of mods on it (over 400 levels). You can also get access to DRM-free copies of previous Betas and Alphas of the game, including very old ones.