The 5th Maxiumus Cup event will reward Tetris 99 players with a Platoon inspired in-game theme. The Maximus Cup is a free event and starts tomorrow, Just 12th at noon PST (3PM EST) through July 15th. From the press release: "If you’re a Nintendo Switch Online member, you just have to play the Tetris 99 online mode during the event period. A greater number of event points are earned the higher you place after each match, and once 100 event points are reached, the new Splatoon theme for your Tetris 99 game is unlocked!"
You need to be a Nintendo Switch Online member to participate. Then play the Tetris 99 online battle-royals style mode where 99 players enter and one is is left standing. The recent Big Block paid DLC also added two offline modes to the game if you're not interested in going for theme or playing online.