cx-newsletter/views/6-manage_groups.php

<?php

namespace cx_newsletter;
use \add_submenu_page;

class manage_groups_view 
extends view
implements view_interface {
    public function connect() : void {
        add_submenu_page(
            'cx_newsletter',
            __('Manage groups', 'cx_newsletter'),
            __('Manage groups', 'cx_newsletter'),
            'customize',
            'cx_newsletter_manage_groups',
            [$this, 'action']
        );
    }

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