Not really sure what the adding-customer-user-providers does not explain? The point of your custom provider is that it has custom code related to your use-case?
- Add a custom service provider which links to your custom class
- change the default provider to your custom provider in auth.php or create a new one so you can switch between the two
- Make sure your custom class adheres to the UserProvider interface
- If you set this up you can use all the normal Auth:: methods, like Auth::user()
And a user-provider alone is probably not enough so also take a look at the custom guards: https://laravel.com/docs/5.8/authentication#adding-custom-guards
But when reading your post I think you actually just want to authenticate with an existing forum user, in that case, you probably want to use this: https://laravel.com/docs/5.8/socialite Take a look at the custom driver website https://socialiteproviders.netlify.com/, you can create your own custom driver for IP-board
IPBoard as a single-sign-on method which is probably what you need. Read the documentation and add your own socialite driver.https://invisioncommunity.com/services/sso/