Now I\'m using something like that for authenticating the user on my base site:
if (Auth::attempt($request->only([\'id\', \'password\']))) {
/
A bit too late to answer, but I've been having a hard time trying to figure this out, so for the sake of completeness and to maybe help others in the future.
If you take a look to this part of passport's code you'll see that it also looks for a validateForPassportPasswordGrant method, so in addition to Alexander's answer, that's how you can authenticate an user using custom fields.
Hope it helps someone.