Referral codes must be entered before purchasing a second parcel after the tutorial. Once you have purchased your second parcel after the tutorial, the option to enter a referral code will no longer be available.

To enter a referral code after the tutorial:
  1. Click on your profile icon to open your game profile.
  2. Click the cog button under your username to open the settings menu.
  3. Click Referral Code.
  4. Enter your 6-digit referral code.