From Steam page
'As a token of appreciation for our amazing community, we are pleased to offer a free copy of Dying Light: Bad Blood to every Survivor eager to expand their Dying Light experience. Everyone who owns Dying Light (regardless of platform) can head over to our docket site to claim their Steam code.
To claim the game visit our docket site, log into your account (if you don’t have one, you can register for free) and link your Steam/PS4/Microsoft account.
If you own a copy of Dying Light, a banner will inform you that you’re eligible for a free Dying Light: Bad Blood Steam code. Banner can take a few minutes to appear. Click the link on the banner and follow the instructions to add the game to your Steam library.'
Lists event time as Tue, 28 January 2020 @ 4:05 am - 5:05 am AEST so probably limited to this time. Update - Just redeemed my code so maybe not. Site must be getting hammered because keeps timing out but keep trying you will eventually get there.