Using Modal window / Lightbox with a button, shortcode or image link
Video Play Button Modal Shortcode
You can use the Video Lightbox Button from the shortcode generator which outputs a nice play icon.
[video_play src="https://www.youtube.com/embed/4AZR8a5XVSs" width="400" size="xl-icon" icon="glyphicons-play-button"]
Great for when you want a button to open a modal with text in the modal window.
[modal title="Modal Title here" button_type="default" button_text="Button Text" button_size="default" footer="off"]Content goes here[/modal]
Video Lightbox from a Button
If you're wanting to launch a video from a more regular button instead, you could use this code:
<a class="btn th-btn btn-cta" href="https://www.youtube.com/embed/4AZR8a5XVSs" data-toggle="lightbox" data-width="1280">Launch video</a>
That outputs a CTA button, you could swap out "btn-cta" for "btn-standard" or "btn-ghost" instead. That code can be added anywhere HTML is accepted. You could just add that into the slider Content Editor box instead of using the fields for a slider button.
Video Modal launched from an Image Link
You can use this code to output an image link that opens a video modal:
<a href="https://www.youtube.com/embed/4AZR8a5XVSs" data-toggle="lightbox" data-width="1280" rel="nofollow"><img src="http://path-to-your-image/image.jpg"></a>