Redirect the Default Registration Page

Sends visitors who open wp-login.php?action=register to your own registration page.

Code

add_action( 'login_form_register', function () {
    wp_safe_redirect( home_url( '/register/' ) );
    exit;
} );

Change /register/ to the path of your registration page.

Install WPIDE on your WordPress site (it is free) and open Code Snippets.
Use snippet above adds it from the library in one click. Or copy the code into a new snippet with the same code type.
It is added inactive: review the code and settings, then switch it on.