Wait, how can a passive relate to a missile being launched ?
What is exactly, the event you want ?
Example:
This passive spell will heal you 30% from the damage by the missile.
Is this what you meant ?
If this is what you meant, you yourself must create the event - more so, the algorithm.
When the missile reaches the target, do the action.
First, you must have a missile movement trigger to handle the missile.
After that, detect where does the missile hits the target located in the trigger.