When in the Black Wasp jet you can switch to the HARM missiles, select a target with R or T and when you fire the missile will steer itself onto the selected target before entering LOAL mode.
The HARM being an antiradiation missile shouldn't be able to lock or steer itself to a target that doesn't have radar emissions active.
It seems that the target gets initially recognized as valid, so for a brief moment the missile will guide itself onto the target's position, however after a brief moment the missile will enter the correct LOAL/Antiradiation mode.

See pictures:

Note however that if the selected target is moving, only the original position will be passed as target to the missile.