You can redeem codes in the Options menu. Codes can contain for example card packs or draft tickets. You can get codes for example through official KARDS stream giveaways on the KARDS Twitch channel.
How to redeem codes
On the home screen, click the Options button in the top-left corner.
Click the “Redeem Code” button.
In the popup box, enter your code and click “Confirm”.
The code will be redeemed and any contents of the code added to your account automatically.
I couldn’t find it and I’m playing mobile.
The redeem codes button is no longer there on mobile