How do I add an Extract Ventilation Rate to a room?

I have a project set up in the Design Database and have carried out the Heat Loss Calculations. I now need to go back and alter a few rooms which have Extract Ventilation only so add this ventilation rate and remove the natural infiltration. I'm sure I've done this before but can't remember how it's done? Can anyone point me in the right direction please?

I can see how to add supply air but not the extract rates.

Thanks

Pete