I no longer use Discord.Ext based plugins, but if you're specifically looking for a Oxide -> Discord then DiscordSync is what you're looking for.
This thread was created due to me getting rid of Discord.Ext and finding a way to do it through the other system I have. I didn't even mean to ask such a silly question in the first place, as my answer was right in front of me xD
as far as "automatically" authing a player, that is not possible. There has to be some form of user interaction whether it be via command in game OR external login where your server talks to both Discord bots and oxide.
You can set your 'default' role permissions in Oxide with the permissions you're giving to "authed" players, but then there is no point to "authing" if you're just going to give it to everyone who joins haha.
Discord Auth/Sync is your best solution it sounds like.