The official Pokémon Information Bureau @Poke_Times Twitter account has posted a new serial code for Pokémon Legends Arceus as a thanks for helping find Lost and Found satchel items in the game. The code is: THX4Y0URHELP. It is active from March 4th, 2022 until April 1st, 2022 at 8:59 JST.

The code gives players:
5 Grit Pebbles (ガンバリのいし)
5 Rare Candy (ふしぎなアメ)
5 Star Piece (ほしのかけら)

Source: https://twitter.com/poke_times/status/1499686115044999173