OR, you can use Research.
If you use Maker's method, the unit won't appear at all in the Barrack menu.
If you use Research, the unit's icon is there BUT it is greyed out indicating that you have to research it first.
It will have a golden text showing the name of the research.
Now, name your research such as "Enter Black Citadel".
And when you hover to your mouse to that locked unit, the requirement should be: Enter Black Citadel
Now, you will know what to do to unlock that unit.
As for upon entering the region, make the research level to 1 and it automatically unlock the unit (I don't have war3 atm, can't show the trigger).