It looks like it works for both custom fields added in the UX and via a plugin. For custom fields, they get names like user_field_1, I’m pretty sure. You can look in the table with data explorer, for example. Or maybe it means only fields added by the UX (“admin-defined profile fields”), in which case you’d use the name you gave it when you created it. I’d just try it both ways.
And github is down, so I can’t look at the code there.