Open your
MDE account, then buy the new tickets from Disney via that account.
The tickets should instantly and automatically show up in your MDE account and
you will have full privileges of the tickets for making advance FP+.
ADRs do not require the purchase of tickets in order to advance book.
Even if your tickets do not show up automatically in your MDE,
you can simply manually add their confirmation numbers to your MDE
and the tickets will become linked to your MDE that way.
-OR-
Using this "manual input" method is also the way to do the connection
so that that you COULD
decide to purchase tickets from one of the many authorized
WDW ticket resellers, and take advantage of the price discounts they offer.