Forget the pissy little key.
Get a high quality 6mm Allen socket that fits the hex snuggly and use a breaker bar to jar it loose. Giving the plug a good rap with a hammer and punch to break any corrosion will help. The quality and fit of the Allen socket is vital. If it isn’t a really good fit you’ll risk rounding out the plug.
As a word of advice I never drain the oil before I’ve got the filler plug loose. At least that way if it won’t come out you can still ride the bike while you work out how to get it out.