You need to join Iberia and then wait for an email invitation. Also bear in mind that you cannot transfer your avios points out of a new Iberia account until 90 days have passed. It's a pain if you want avios now, but in the long run, it's well worth it.

Also worth mentioning you can earn 3k avios per month with Iberia E-Rewards rather than 2K with the other avios partners.