Below is from the Broan installation guide.
It says to use the supplied cord and plug.
The only code issue I can think of is that it might only meet it's UL listing with the supplied cord and plug.
If you want to be certain that hard wiring will not void the UL listing, you should call Broan and ask them.
Note: This range hood must be properly grounded. ALWAYS SWITCH OFF THE ELECTRICITY SUPPLY BEFORE
CARRYING OUT ANY OPERATIONS ON THE APPLIANCE.
The unit should be installed by a qualified electrician in accor-
dance with all applicable national and local electrical codes.
GROUNDING INSTRUCTIONS
This appliance must be grounded. In the event of an electrical short
circuit, grounding reduces the risk of electric shock by providing an
escape wire for the electric current. This appliance is equipped
with a cord having a grounding wire with a grounding plug. The
plug must be plugged into an outlet that is properly installed and
grounded.
WARNING - Improper grounding can result in a risk of electric shock.
Consult a qualified electrician if the grounding instructions are not
completely understood, or if doubt exists as to whether the appliance
is properly grounded.
Do not use an extension cord. If the power supply cord is too short,
have a qualified electrician install an outlet near the appliance.
Position the electrical outlet at a maximum distance of 33-7/16” (85
cm) from where the lead exits from the hood (see illustration along-
side).
Fit the plug into the outlet.
