Originally Posted by
threeoh
As others have said, to get the free checked bag you have to use the card to buy the ticket.
You also have to have your MP number in the booking to get the free checked bag. So, if you'd earn much more miles on another carrier, you have to check your bag first before you change the frequent flyer membership on your booking. (This is the opposite order to the order the kiosks present those options, or at least it was last time I did it!)