Shopware 6 plugin override footer

I am developing a plugin in shopware 6 and I want add code to the footer when the page loads. I've tried several things but it doesn't work for me. How should I continue?

use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Shopware\Storefront\Event\StorefrontRenderEvent;

class MyPluginFrontendSubscriber implements EventSubscriberInterface
{
    private $twig;

    public function __construct(\Twig\Environment $twig)
    {
        $this->twig = $twig;
    }

    public static function getSubscribedEvents(): array
    {
        return [
            StorefrontRenderEvent::class => 'onRenderFooter'
        ];
    }

    public function onRenderFooter(StorefrontRenderEvent $event)
    {
        $template = '@MyPlugin/storefront/layout/base/customjs.html.twig';
        //I dont know to do here

    }
}

public function onRenderFooter(StorefrontRenderEvent $event)
    {
        $this->twig->addGlobal('shopware', array_merge(
            $this->twig->getGlobals()['shopware'],
            ['footer' => $this->twig->render('@MyPlugin/storefront/layout/base/customjs.html.twig')]
        ));
    }

Dont work

You can extend a template by matching the path of the template and using sw_extends.

<PluginDir>/src/Resources/views/storefront/base.html.twig

{% sw_extends '@Storefront/storefront/base.html.twig' %}

{% block base_body_script %}
    {{ parent() }}
    <script>...</script>
{% endblock %}

If it's just about adding custom javascript also consider this way to go about it.

For adding variables to be used in the template programmatically, see this part of the documentation.