These codes are sent by email.
Check out this helpful post from Spotify if you did not get one:
1: Make sure you postal code is up to date and you have the correct email entered https://www.spotify.com/account/profile/
2: Go to the concerts section under Browse in the app and make sure to select your city
3: Make sure you have "Spotify News and Offers" and "concert notifications" turned on to get emails https://www.spotify.com/account/notifications/
If you had these off, turning them on won't send you the code right now, but it will make sure you get presale codes in the future.