Laravel 5.5 How can I get the accessToken of a custom user?

I can access the personal accessToken of a loggedIn user by

$user = Auth::user();
$user->AauthAcessToken();

But for a custom user i tried for the personal accessToken as following but gives null.

$user = User::where('id', $id)->first();
$user->AauthAcessToken();

How can I get the accessToken of a custom user?

via Chebli Mohamed