If you look in quests.xml, the rewards are drawn from loot groups (not loot containers).
If you want to add your own loot group, then create one in loot.xml, and reference it from whichever quest(s) you want.
On the other hand, if you just want to add an item to quest rewards, then add it to whatever loot group is used by the quest(s). Those start with "groupQuest", and there are a few of them, and not all quests use all those groups.
Make sure you read through the comments at the top of quests.xml to understand how it all works.