Not sure if this is common knowledge or not, but figured I'd post.
I just booked an economy award ticket on United - flight is a TAP flight. After I booked, I went to the TAP website to choose seats, and it allowed me to choose a seat in the preferred section for both legs of the flight, for free (although the seat map shows that those seats should incur an extra charge).
I called TAP to ask about that, as I didn’t want to be charged a fee when I check in at the airport, and they said that because it was booked with miles, those preferred seats can be reserved at no extra cost.