How to redirect new registrars to a custom registration page instead of WP default registration page?

Since wp-login.php control both login and registration, it is better to redirect the visitor to a login page from wp-login.php.

You can use this code for redirecting your registration page –

add_action('init','custom_registration_page');

function custom_registration_page() {
 $new_registration_page_url = home_url( '/register/' );
 global $pagenow;
 if( $pagenow == "wp-login.php?action=register" && $_SERVER['REQUEST_METHOD'] == 'GET') {
 wp_redirect($new_registration_page_url);
    exit;
 }
}

This code will redirect user registration to yourdomain.com/register/. You can point to your desired URL on the 3rd line.