From the announcement (note: all references to "this Facebook post" means the one on Corvus Belli's page) :
Happy German Unity Day! We draw 1 Shikami (Combi Rifle)!! To participate, you just need to comment on this Facebook post, or on Twitter using the hashtag #InfinityShikami answering the following question: “How will you use it? “ Everyone who post a comment during today, October 3rd, will participate in the draw. Tomorrow, We'll announce the winner (October 5th). Good luck to everyone!