IN-GAME

On main menu, select the three bars on top right.

Then press Settings. After that scroll all the way down until you see Gift Code option.

And input any codes that go there, once inserted they'll be sent to your Mailbox.

ON-SITE

You can also gain codes from the main website of SEGA. (You will require an account in order to get your own exclusive codes, once you've also linked your account to whatever platform you're currently on. Click on Rewards on top center, then click Claim Your Reward as soon as it redirects you to another page.

Once you've created account and linked yourself to the game your prizes await on this page. Redeem them and codes will be shown. You can do the rest inside the game.