add any image URLs to a lightbox gallery (page settings > custom code > footer scripts)

footer-scripts.html

<script>

document.addEventListener('DOMContentLoaded', () => {

    const lightboxSelector = '.my-gallery-lightbox'; /* change this to match the selector for your lighbox element */

    setTimeout(() => {
        const lightboxElement = document.querySelector(lightboxSelector);
        if (!lightboxElement) return;

        document.querySelectorAll(
            (JSON.parse(lightboxElement.getAttribute('data-x-lightbox'))).linkSelector).forEach(button => {
            const galleryStr = button.getAttribute('data-gallery');
            if (!galleryStr) return;

            button._glightboxItems = galleryStr.split(',')
                .map(url => url.trim())
                .filter(url => url.length)
                .map(url => ({
                    href: url,
                    type: 'image'
                }));

            button.addEventListener('click', () => {
                xDynamicLightbox.Instances[lightboxElement.getAttribute('data-x-id')].setElements(button._glightboxItems);
            });
        });
    }, 500);
});

</script>

to your button, add an attribute data-gallery with a list of image URLs seperated by commas.