At present, if you own a DLC 5x5 code for a promotional item, this code can only be claimed through the Xbox and UWP (Microsoft Store) versions of Sea of Thieves and not through Steam.
Should you own Sea of Thieves on both Steam and UWP / Xbox, the way to receive these items within the Steam version is to redeem the code through the Microsoft / Xbox stores and then launch your UWP / Xbox version of the game.
Once your game entitlements have been confirmed (this occurs when proceeding from the 'Start Game' screen to the main menu) the items will now be available in Steam as normal.
Should you own Sea of Thieves on both Steam and UWP / Xbox, the way to receive these items within the Steam version is to redeem the code through the Microsoft / Xbox stores and then launch your UWP / Xbox version of the game.
Once your game entitlements have been confirmed (this occurs when proceeding from the 'Start Game' screen to the main menu) the items will now be available in Steam as normal.
