you dont need no triggers for this.
you need an active ability. you make that ability an item ability. you set a mana cost for this ability.
then you add this ability to the item you like. you set this ability to be the first ability in the list of abilities the item posseses.
should work.
if not there is some boolean in the item itself which needs to be set which i forgot right now. long time since i worked with items.
I believe if you put the ability as the first bonus that the item gives it should show mana. You sure it actually requires mana?