The comic can be acquired by reserving either the ultimate or standard edition of the game from Amazon. You can do so with this link from the shopping website. You can also check out the full description of the preorder bonus from the online retailer below:
Introducing Supergirl to the world of INJUSTICE! Years before she came to Earth, and long before she became Supergirl, Kara Zor-El was like any other girl growing up on Krypton. But when Brainiac’s forces attacked the planet, her family put her in a rocket and sent her away – along with her cousin, the infant Kal-El – to ensure that the heart of Krypton would never stop beating. This is her story, and it puts her on the path to discovering the truth about her cousin, Superman, in the pages of the INJUSTICE 2 comic book series and the upcoming video game sequel.
Injustice 2 is releasing on the 16th of May in North America for the PlayStation 4 and Xbox One consoles.