Triggering Unit is what you're looking for.
Trigger:
- TargetYourUnit
- Events
- Unit - Unit1 <gen> Is issued an order targeting an object
- Conditions
- (Target unit of issued order) Equal to Unit2 <gen>
- ((Region centered at (Position of Unit2 <gen>) with size (800.00, 800.00)) contains Unit1 <gen>) Equal to True
- Actions
Is this what your looking for?