Join Date: Aug 2011
Location: Barcelona, London, on a plane
Programs: IB+ Gold, TK E+, Hyatt Globalist, Marriott LTP, IHG Diamond
Posts: 14,210
When providing passenger information, there should be a box "member of Iberia Plus or another loyalty card" or something like that.
The most important thing is to not buy the ticket when logged into Iberia Plus. In my experience, it then becomes a pain to remove your IB+ number and replace it with something else - the system keeps reverting to the original IB+ number.