In early March I bought a limited edition Flower & Garden Festival MB for my DGD (who wasn't traveling with us that time) to use on our Spring Break trip. At the same time I bought a special one for myself. The CM in the shop activated mine right there, and I used it the rest of that trip without problems. When I got home I linked DGD's MB to her on MDE (I manage her account), and at Spring Break it worked perfectly well. So if you buy one online prior to the trip, you can link it yourself and it will be ready to use when you get there.
To link DGD's MB, I used my PC as opposed to the app. Under My Account I clicked on Magic Bands and Cards, which brought up a list of people whose accounts I manage. At the bottom of that page there's a Link button that you use to link a purchased MB to its new owner. It was very easy - except for reading the tiny numbers/letters on the MB! I had to get out the magnifying glass!