Stupid Question, Build on Behaviour

[fischy33]

Hey guys,
i want to build my towers on another unit.
i read that therefore i have to use the Vespene Geyser Behaviour, but i am not able to make a archon being build on top of an supply depot...
i only was able to make an builder, so he can build an archon on a bugged vespene geyser...thats not what i want...anyway, if you are bored maybe you could explain me, how to make an ability, which drops the archon like a mule, but only if you clicked on a specified structure....

 

[Dr Super Good]

Maybe a validator that checks unit type could be used.