you would open the containers parent ref id and add the items to it, but that would add items to all the chests with that id. Not sure if they made a new way of doing it, but I used to make a new container ID for the items I wanted to only be in that one chest.
Ok I got it. It seems to be changed now. You just double click the chest, click Edit Base and then right click under item list and select New. It'll add the potions to the chest in the desireable amount. I checked other chests and there were no potions in them, so I guess now each chest has it's own ID.