User Manual LG LMHM2237ST 2.2-cu ft Over-the-Range Microwave

Yes the vent can be run anytime. There is even a timer that can be
programed to turn it off after a designated time.

The 2.2 cubic foot Over-The-Range Microwave with the Sensor
Cooking Controls, Model # LMHM2237ST does not provide any
way to vent it directly outside. It is recommended that the area
under the hood be kept properly vented by turning the fan on and
keeping it clear of steam and smoke

There aren't any trim kits available for the LG LG 2.2-cu ft Over-
the-Range Microwave with Sensor Cooking Controls (Black
Stainless Steel), Model # LMHM2237BD. This unit is designed to
be installed between cabinet openings above a range oven
under upper cabinets. The microwave oven should not be
mounted in an island or peninsula cabinet.