Referral code automatically applies with referral links
When you sign up with a referral link, the code applies automatically.
You can apply it as following
Find the My settings block on the Settings page. If it's not there, you can't change the code.
You can paste the referral (tracking) code into the corresponding field and click Apply:
How to check if the code is applied?
To check if you (or your referral) have correctly applied the referral code you can check the same box on the Settings page: