Enable Helpdesk access only for users with a specific product

Add this code to site.php file:

function isHelpdeskEnabledForCurrentUser()
{
    $user = Am_Di::getInstance()->auth->getUser();
    if (!$user) {
        return true; // Give the user a chance to log in first
    }

    foreach ($user->getActiveProducts() as $product) {
        if ($product->data()->get('helpdesk_enabled')) {
            return true;
        }
    }

    return false;
}

Am_Di::getInstance()->productTable->customFields()->add(
    new Am_CustomFieldSelect(
        'helpdesk_enabled',
        'Enable access to Helpdesk',
        '',
        '',
        ['options' => [0 => 'No', 1 => 'Yes']]
    )
);

Am_Di::getInstance()->hook->add(Am_Event::USER_MENU, function (Am_Event $event) {
    $menu = $event->getMenu();
    if (!isHelpdeskEnabledForCurrentUser() && ($page = $menu->findOneById('helpdesk'))) {
        $page->getParent()->removePage($page);
    }
});

Am_Di::getInstance()->hook->add(Am_Event::INIT_CONTROLLER_PAGES, function (Am_Event $event) {
    $controller = $event->getController();
    if (!($controller instanceof Helpdesk_IndexController)) {
        return;
    }

    if (!isHelpdeskEnabledForCurrentUser()) {
        throw new Am_Exception_InputError(___('Access denied!'));
    }
});

By default, Helpdesk access is disabled for all users. To enable it for a product, edit that product in aMember CP -> Manage Products and set Enable access to Helpdesk to Yes in the Additional section.