I think they will start sending out codes immediately after the official release, starting on 05/19/2026, and they indicate September as the month during which you will still be able to participate in the promo and that September is the last month when you can send a receipt and receive a code in return.
p.s. By the way, take a look at the topic called Promos & Collabs DLC should be more accessible! and if you agree, then vote.