A promo code gives you the voucher of a promotion: a discount on trading fees for a period, or a cash bonus paid once you meet its conditions (What are vouchers and how do they work?).

Where to enter it

Signed in: Account → My vouchers, field Promo code, then Redeem. The voucher appears in the list at once.
At registration: click Have a referral code or promo code? in the sign-up form and enter the code in Promo code (optional). The code is applied once your account is confirmed.

Why a code may not work

The code is invalid: a typo, or the code is no longer active.
The code has expired or all activations are used: a code is valid for a period and for a number of activations.
You already used this code: each code works once per account.
Only for new accounts, Verified accounts only, You already have this promotion's voucher: the promotion's own rules.
Too many attempts: wait an hour and try again.

Codes are case-insensitive. A code entered at registration that could not be applied is simply skipped; you can try another code later in My vouchers.

How do I use a promo code?