As mentioned punch a small hole through the rubber grommet in the firewall. It will be toward the bottom of the upper tier in the frunk, to the left or right of the regular battery location. Once you punch the hole use a screwdriver to push through the fabric barrier that lines the interior. Then look up under the dash to identify where the wire will be coming through. Once you finish wiring everything I would recommend putting a little sealant on the frunk side of the grommet to keep water from getting through.
EDIT:
Found two threads with pictures of the grommets:
Best way to wire an amp in the frunk?
Running wires through firewall