It has been discussed repeatedly in several threads; just do a search. Any 12 gauge 12-3 cord will work without risk of overloading it and it getting even warm. The Honda OEM Charger only pulls 12 Amps max ans many 15 Amp circuits use 14 gauge wire.
Odobo is right on about protecting every connection (wall receptacle if outside and between ext cord and charger plug) from water and of course don’t set the “brick” on the ground.
Remember that ext cords are not allowed to be in continuous flex or run through walls or be pinched by things like doors and windows. I charged w a 50 ft 12-3 ext cord for a month before my Level 2 was installed and it never even got warm.
I have pix of a NEMA approved cover for the outlet if it’s outside and one of a cover for the connection between ext cord and charger if you need it.
You also might want to consider a small lock for the hole in the charging handle to discourage snatch and grabs of your charger.