How to add a custom Authentication Provider

[softrare]

I have a project taken over from someone who tries it with a custom compiler pass:

public function process(ContainerBuilder $container)
{
    $definition = $container->getDefinition('security.authentication.provider.custom');
    $definition->setClass(CustomAuthenticationProvider::class);
}

but it doesn't work, meaning the CustomAuthenticationProvider isn't called. How to do this?