Do you mean: you want to buy a box (it has some foods) and you will take it to another place?
You can say: Cho tôi mua một hộp mang về !
Cho tôi mua một hộp (cơm/ thức ăn) mang về!
The seller will ask: Bạn muốn mua những gì? (it means: what kind of foods do you want to buy)
In Vietnam, we want to buy a lunch/dinner box, it includes: rice and some foods which you want to eat.
In other case, if you want to buy something and take it to your home/school or another place, you can say: "Cho tôi mua + quantity (1 or 2 or 3,..) + unit (box, sheet, pce,..) + (type of things which you want to buy) + mang về! "