Put a empty, dim show Text command infront of the choice command.
the choice window will adjust its position accordingly to the textbox window. So you can have them displayed up or down.
The choice Box also uses the dim background of the textbox thisway.
Its up to you if you include text or not, both will be displayed at the same time.
Its not obvious that this works.
Dim Textbox + Show Choice = Both at same Time, standard in rpg maker.