Hi
If you are referring to the security code that must be entered when adding a card to Apple Pay, then this is the last three digits on the signature strip on the back of your payment card (or the four digits on the front, for American Express cards).
If you are instead referring to a verification code that may be required when adding a card to Apple Pay, then contact your card issuer. Verification codes are sent by the card issuer (not Apple) to the contact details that the issuer has on file for you.