All you do is, in the contact box, put the morning of your booked appointment. So - if you've got a booking for 2:00 on the 28th, put "contact me" as the 28th, between 9:30 and 1:00.
I don't really see the point of this box either, but Aw won't change it because they can't be fucking bothered, and there's no need for confusion anyway. Does the girl in question speak good English btw?