Yes everyone staying onsite gets the MB. After you book you will have to link your resort reservation yourself to your
MDE account.
You may also have to be more proactive getting your Magical Express documents (if your using it) as the booking site technically owns your reservation not you. You will also have to be sure and get your ticket #'s so you can also link them to your MDE and make your FP+ at 60 days.
Honestly it would have to be an amazing discount for me to book using a 3rd party and give up control of my reservation.