You need to fill the barrel with severed heads first. Not sure if the command does anything then or not as I've not had a chance to try yet, but all you actually need to do is trigger a quickening near a barrel with severed immortal heads in it, so you can just set up a stockpile that only allows immortal corpses around it.

I think you only need one, but I assume results are better if its full.

Old post I know, but I'll leave it here so my other comment isn't buried.
You need 10 heads in the extractor and then you can command a pawn use an immortal marked with the extraction trigger. Also if the extractor hits 100% before a trigger is used you will only be able to empty it in the current version that is.