Square Enix requires a product code to register this game. The page talks about a box, and there ISN'T any box—it was a Steam purchase. Alternatively, it (the SE site) says that the code may be found on a page in the manual—and again, there ISN'T any "manual;" the so-called manual link in the Steam browser leads to a PDF file whose sole content is a graphical representation of the Linux Launcher," which is absurd, since I'm running Hitman on the Windows machine I purchased it on.

There's supposed to be a way to reveal Steam codes in the Steam browser, but I can't figure it out for this game (I've made a relevant post there regarding the matter). Maybe someone here can help, even if the answer is, "You can't register this game on the Square Enix site."