The alphanumeric will work as well to link to
MDE. Some have reported that their original UK 6 digit number eventually works too, although I have never done this and can't comment on it.
If they can give you an alpha numeric reference, take a note of it. Once you link your reservation to MDE and your option to customise MB's shows up, if you look above the MB's your 12 digit number is there.
You want to take a note of this before you do anything to the bands. As soon as you customise them, you can't see it anymore.