See it in action

Choosing the metafield (version 1.3.0)

The See it in action block reads this content through its Media lightbox metafield setting. The default key is custom.media_lightbox.

Keep the default key above and it works with no extra setup.
Or rename it: create the metafield under your own namespace.key (for example, your brand name), then set Media lightbox metafield to match.
Reuse on one page: create one metafield per copy (for example custom.media_lightbox and custom.media_lightbox_2) and point each Media lightbox metafield at its own key to show different content.