What's the best way to repair a hardi-plank cornet?

You could probably get a wood or vinal to match that and install instead

If you cold find a piece of scrap somewhere, maybe a l umber yard, glue it on and then paint the color to match.

Clean work area inside and out with rubbing alcohol. Sand rough surface of impact area w/ block sander.

Then wedge beaded styrofoam blocks into the bottom of the hole & the top of the hole, 1/2” below & above. If any holes inside this plugged area, caulk them.

Wipe any loose styrofoam beads off the inside with alcohol.

Get out 2” or wider blue painters tape. And piece together a giant band aid that will wrap around the hole, with 2” of overlap on all sides. Attach the band aid over the outside of the hole at the bottom and have someone hold it at an angle away from the hole access, while you spray Spray Insulation Foam into the hole, don’t overfill and then put the band aid all the way on, burnishing the edges.

Buy Non-Over Expanding spray insulation foam, water clean up, if you can find it. Otherwise, way underfill the hole.

After it cures 24 hours, take off the tape and clean up the residue by gently block sanding and then wipe down w/ alcohol again.

Gently press the foam to see if it compacts anywhere. Sand the surface to be 1/16” or more below the adjacent trim.

Mix up some Bondo and work it into the foam and smooth it very well. If it’s too hot of a mix, stop, let it dry and make another batch.

After the Bondo dries, sand smooth, and look for spot putty application places. Sand & prime when dry. Don’t forget to make sure everything is still grease & dirt free at each step. It’s outside in the Fall.

Paint to match two full coats the entire trim board.

Your best bet is probably to replace the corner pieces instead of trying to patch it. I have worked with Hardi-Plank many times and it is very easy to work with.

Thanks to all for your great answers!

I would try to find a small piece at the home store that would patch the space that is damaged.