Doesn't matter which you do.
Linking works the same way whether you do it before or after you make your FP+ selections.
You just use the code on the back of the band to link it to your account.
Just be sure you link it to the proper person because once its linked its linked.
The key thing is that you have your tickets linked before your FP+ selections can be made.
As pp have said, as long as you have a ticket linked to your MDE account and can make your FPs, you can link the MB anytime. I would do it as soon as you get the MB, just to be sure it linked correctly and MDE recognizes it.
We stayed at the Dolphin four years ago when magic bands were brand new... we made our FP at home and purchased bands in HS on our arrival day (we did a half day in HS). The CM at check out linked our bands for us.
Flash forward four years... we just returned a couple of weeks ago from a trip staying again at Dolphin, used our same magic bands from four years ago. I bought some cute decals from etsy and we had a magic band decorating session at home before our trip. A lady on a bus one morning asked me which special version was my band because she wanted to buy one- I had put the peace, heart, Mickey Mouse decals on mine.
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