Iberia appears to be running a promotion until Jan 22 offering a 25% discount on booking using Avios.

The fine print in the email I got is:
Purchase period: 02/01/2024 - 22/01/2024
Travel period:
Destination Spain and Europe: 02/01/2024 – 31/12/2024
Flight periods not included in the promotion: 09/01/2024 - 20/01/2024 ; 14/03/2024 - 01/04/2024 ; 30/04/2024 - 06/05/2024 - 22/06/2024 - 15/08/2024.
*25% discount on the price of the ticket paid entirely or partly with Avios when you select 'Discount with Avios' on www.iberia.com. ​
Only valid for members of the Iberia Plus programme for purchases made between 2 and 22 January 2024 for flights operated by Iberia, Iberia Express and Iberia Regional Air Nostrum, to fly on any date and in any class or cabin.

Question is: Does anyone know how you select 'Discount with Avios'?

I've tried sample bookings and see no place in the process to make this choice. The prices are the same as they were before the promotion,
so I suspect I'm missing something. On the other hand, this is Iberia IT potentially at work also.

Thanks for any tips.