You cannot. Effects created on a unit cannot have their positions changed.
The best you can do is create an effect at location instead of on unit. You can use unit's position as base and offset the effects location (x, y, z coordinates) to kind of get the area in front of its face.
It has two major drawbacks, though:
- The x/y/z offset is different from model to model and must be determined manually via trial and error. There is no convenient way which would give you the position of, for example, "head" attachment point.
- It will look strange during certain animations the model will be playing. For example Blood Mage moves his head a lot during his various attack/spell animations. As there is no way to get "head" attachment point's coordinates, it will be very hard for you to reliably calculate the offset yourself.