Batman Arkham Collection and 3 Batman Lego Games - Epic Games

Visit here:

https://www.epicgames.com/store/en-US/collection/batman-free-week

Create an account or log in to already existing one, and permanently add the games on your account.
Alternatively you can redeem it from the Epic Launcher on the game's giveaway page.

Click on each game and add it to cart and checkout

(its still new they are still updating the pages, it may be buggy)