If you have received a promotional code for Dauntless, you can redeem it by logging into the Dauntless website at https://playdauntless.com/ and visiting the "My Account" page.
Enter your code in the redemption box and hit redeem code. After this, your item should be unlocked in game. Please note that you may need to visit the core breaker to receive your items.
Additional Note: Please make sure you're logged into the correct account. Many people have multiple Epic accounts, so it's important to double-check that this is the one linked to your Dauntless Account. When in doubt, try logging in through PC even if you play on console. If you're having trouble locating your account in this way, please submit a support request below and be sure to include your display name in-game as well as which platform you normally play from.