cx-newsletter/views/4-manage_customers_view.php

<?php

namespace cx_newsletter;

class manage_customers_view 
extends view
implements view_interface {
    public function connect() : void {
        \add_submenu_page(
            'cx_newsletter',
            __('Manage customers', 'cx_newsletter'),
            __('Manage customers', 'cx_newsletter'),
            'customize',
            'cx_newsletter_manage_customers',
            [$this, 'action']
        );
    }

    public function process() : void {
        $this->add_activity(manage_customers_activity::get_name()); 
        $this->add_activity(import_customers_activity::get_name()); 
    }
}