A few days ago, the Hitman franchise celebrated its fifteenth anniversary, and today Square Enix launched a promotion for the occasion, just announced with a press release.
Update: Unfortunately the North American link is down at the moment of this writing, probably due to intense traffic, but the promotion will run until November 29th. Most probably the page will go back up before then.
In order to get it for free, you need to use the code “FreeSilentAssassin” at checkout. You’ll also be required to log in (you need to create an account in order to do so).
On top of that, until November 29th, you get a 10% discount on pre-orders of the upcoming Hitman game on PC only.