What wiring is needed for a propane unit heater in a garage?
Cover your bases. Install a 20A receptacle in a metal box. If the breaker is GFCI then use a standard receptacle. If the breaker is not GFCI then put in a GFCI/receptacle. Since this is a dedicated circuit, ideally you should install a single receptacle rather than the typical duplex.
If the installers want to plug in with a cord, all done. If they want to hardwire, they can do that in the same box. Either way, you've got everything they could need already in place. Worst case, you've spent a few dollars on an extra receptacle that isn't needed, but that's better than the opposite problem of having to pay extra for a second installation trip.