If you add it in at check-in there is a fair chance it won't take, I fear, so best done in advance. I wonder if Iberia has a slightly different passport number, but I guess you could try ringing up BA to see if an agent can force it through (possibly they won't be able to if this is essentially an IB booking).
The important factor is that it is on your booking, in the top section of MMB, but it's not in the BAEC profile information. If you put in there before OLCI then the self printed and check-in desk printed boarding passes with have Pre Check clearly printed on them (unless you're randomly sent out to Priority). However it still doesn't show on the App boarding pass. In the future both the profile and App are supposed to be enable for Known Traveler / Pre-Check.
You are looking for something like this:
