The discount should appear automatically in your cart if you add at least 2 Aquaman tickets to the cart.
Good on Atom website, as well as Atom app.
"Buy 2 Tickets to AQUAMAN and Get $5 Off Your Order!
Offer is valid from January 10, 2019 (12:01 am PT) through January 13, 2019 (11:59 pm PT), while supplies last."
Limit is 1 $5 off on this deal per account.