BA cancelled my flight LPA - LGW and rebooked me on LPA - MAD - LHR tomorrow.

They also cancelled my hold luggage and I have now checked in for my flights but I want to add a suitcase for the hold but I just can’t work out how to do this on the Iberia website or the app.

Can somebody advise how to do this and how to add my BAEC number to the booking as well. I see I have been demoted from Group 1 to Group 4 !

many thanks in advance