Referrals and other promotional credits are deposited into your designated "Payment Account". A Payment Account must be an individual or joint taxable brokerage account (funding not required). A Payment Account cannot be a retirement account. Both you and anyone you refer will be required to have an open brokerage account to receive referral credits.
You can change you Payment Account at any time from Settings > Payment.
Without a valid payment account you will not be able to receive referral credits.