Also if the coupon doesn't redeem for some strange reason, try FREESILENTASSASSIN (in all caps) instead, and it should turn the price to zeros.
Since it was a DRM-free copy this time, I couldn't resist not having an extra downloadable copy of the game. I had nothing to lose.
I just did it, and there is no downloadable file, just a Steam key. And I already have the game on Steam.
So, if anyone would like a Steam key, please reply here.
What happened is that when I punched in the coupon and made a checkout, and then confirmed what's on the basket, SE took me next to a website where I could download a file, with a mention below that most of SE's games are DRM-free (what a lie), and instructions on how to download it. Mainly download the program, then run the program to let the downloader do the work, and don't rename the installer file. They also mention that if a game needs a serial that it would be included.
Also going to the Square Enix profile, I have two entries: Final Fantasy VII and Hitman 2 Silent Assassin. The former has "Steam" on it (it was gifted to me on Steam back before I turned on the service), the latter has "PC" on it. But anyways, I'm not really much missing out if it happened to be a Steam code in the end, as I still have my DRM-free Hitman Collection box with the first four games, completely free of even CD-check DRM (which I tolerate).