Where to enter referral code

I was referred to Fizz by a friend and he gave me a referral code... I signed up and ordered a SIM card and am waiting for it to arrive in the mail, I did not select a plan yet I just ordered the SIM card today..
Where do I enter the referral code because I was never asked for one when doing the initial signup, will I be asked only after selecting a plan and before paying for the first month or.. ?


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