I have just achieved this on my site – example of it working for a picture: My test
There are 2 main steps towards getting it to work.
Firstly, in the plugin options you need to tick the “Add Inline Lightboxes” option. A new section will be displayed at the bottom of the page. For one of these options, choose to ‘Auto Open’.
Secondly, you need to add the code for the Inline Lightbox. This has 2 parts. Part 1 is the stuff you want displayed on the page itself (so maybe you want the video to be displayed there too, or if you want nothing, then use a 1×1 image which is essentially hidden). Part 2 is the inline part which is displayed in the lightbox. It consists of a hidden outer div, and an inner div that contains the content.
This code makes a video pop up from (what looks like) nowhere. If you want the video to be on the page to, simply replace the image with the video’s code:
<a class="lbp-inline-link-2" title="movie" href="https://www.youtube.com/embed/grtJlJK7p5A"><img alt="" src="https://govegan.org.nz/pictures/blank.jpg" width="1" height="1" /></a>
<div style="display: none;">
<div id="lbp-inline-href-2" style="padding: 10px; background: #fff;"><iframe src="https://www.youtube.com/embed/grtJlJK7p5A" height="500" width="500" allowfullscreen="" frameborder="0"></iframe></div>
</div>