HEMS not responding to Ambulance dispatch

I don't think it works like that. HEMS is in it's own separate group, though you might be able to make it work if you use the "Own Vehicle Category".

Quote from Slarow

My understanding from the forums (which may be out of date but I don't see any sort of documentation where I could find a concrete answer to this) is that an HEMS should be selected when an ambulance is part of a response and the HEMS is the closest. This doesn't seem to be working for me.

I have an "alarm" response with a single Ambulance, and I would like the HEMS to be selected when it is the closest ambulance and available

I'd suggest making use of the new alarm and response regulations update.

Give your HEMS the same custom category name as the ambulances and then make a new ARR button using that category.