While it would make sense to have the confirmation. It would be annoying as hell to have it pop up every time you were attacked.
There could be a separate area to configure reactions.
In this area we could say:
Trigger this spell if enemy hp above x
Trigger shield if my hp below x
Some things are already filtered in the spell/skill like to react against mellee or ranged or certain spells.
Imo I think this would be a good solution, though require a little bit more work.
