Embedding Video from a Custom Field

I have theme that uses a custom field to simply hold the URL of a youtube video, and then wrap it in the proper embed code to display it on the page, rather than in the main post content area.

What shortcode do I need to put in the Tumblr Post Text Format field to reference that video, so that it doesn’t default my post back to a Text post?

https://wordpress.org/plugins/social-networks-auto-poster-facebook-twitter-g/