To redeem the Emblem:

Log in or create an account on https://www.bungie.net/en/User/coderedemption
Click the Battle.net button and log in with your credentials
You will then see a screen to copy/paste your code into
Once redeemed, you will be able to access it in-game: Launch battle.net and log into the same account
Go to any Hub and open your Vault; it will be waiting for you in the Collections tab, Emblems section.