If it is advised to buy tickets for popular museums months ahead (Anne Frank museum), then I assume we cannot use the museumkaart (temporary card) since you can only buy it in person when you are in Holland?
You can reserve a time to enter the Anne Frank House while not having the museumkaart yet.
When you check out choose the museumkaart option.
You can always reserve tickets without being in possession of the card yet. No museum requires something like a museumkaart number for reservation.
Caveat: some museums might display the "museumkaart" option only on their Dutch versions of their websites. The Van Gogh museum was like that, though I think they later changed. If that is the case (you see no Museumkaart option on English menu), open a Dutch version side by side to English, and then navigate on both.