Assign Selected roles to user upon registration

Hi, I want to ask how to assign roles when user register, I know it'll use something like this for example:

$user->assignRole('user');

but how can I make it into something like this:

1. I make a page, lets call it setting to define "New User Role"
2. User register by the selected roles from the setting page Also, I use laravel spatie permissions. Thank you.