https://wiki.esoui.com/EVENT_INVENTO...LE_SLOT_UPDATE
This should be the event that triggers if any item get's into your BAG_BACKPACK (from loot etc.) but be carefull, it also triggers if anything happens with the items like durability of worn items changes, or stack is split in inventory, etc.
So add event filters there to "isNew" e.g.. Event filters are described here:
https://wiki.esoui.com/AddFilterForEvent