If you have already bought your ticket and forgot to enter your Club Iberia Plus card number, you can add it by accessing your bookings under Manage your booking. When you reach your booking details, select Passenger information and enter your loyalty card number. Alternatively, you can add it during check-in.
Once you have flown, process it through My Iberia > My Club Iberia Plus > Avios and Elite Points > Claim Avios.