I am trying to enter my IB Plus membership number while trying to book a Vueling flight, and it keeps saying "Incorrect Format" when I enter my IB Plus membership number in "IBxxxxxxxxx" format, "x" being a number. I also tried "IB00000xxxxxxxxx" (leading 0 added) as some systems seem to have done. Still no luck.

Could someone please tell me what I am doing wrong here, and what format it should be entered? Thank you for your help!