echo elgg_view('output/url', [
         'text' => 'Open iframe lightbox',
         'href' => 'https://elgg.org',
         'class' => 'elgg-lightbox-iframe mll',
         'data-colorbox-opts' => json_encode([
               'width' => '80%',
               'height' => '80%',
         ]),
]);

Thanks @Nikolai Shcherbin,

But I want to load an ajax view using elgg-lightbox-iframe, not a url.

Regards,

What do you mean when you talk about elgg-lightbox-iframe?

Have you learned the docs: 1, 2 ?

Also, look at Theme sanbox -> JavaScript tool in Elgg developers tools plugin to view how lightbox works.