I don't think you can do that in your interface. However, there are a couple of things you can do:
(1) The Insert Pages dialog defaults to inserting the last master page you inserted. So if you used Master B for your previous page, it should stay the same when you insert the next.
(2) You can set a keyboard shortcuts in Edit > Keyboard Shortcuts for Insert Page. It's in the Panel Menu product area.
Perhaps someone has created a script to set the default.