Container items

We are not able to fill a container by default?
Add 8 slices of pizza to the pizza box? then when you get a pizza box you have those 8 slices already?
Any help with this?
I used this link, but did not work : [Prefilling Item Containers](https://gist.github.com/Zackaery/879ea14969cf23abd6e0e12d204084c3)