composer config -g repo.packagist

d:\wamp64\bin\php\php5.6.25>composer config -g repo.packagist

  [Symfony\Component\Console\Exception\RuntimeException]

  The "-?" option does not exist.

config [-g|--global] [-e|--editor] [-a|--auth] [--unset] [-l|--list] [-f|--file FILE] [--absolute] [--] [<setting-key>] [<setting-value>]...