You can do this by:
Going to the YouTube video that you want to add to your listing and click on the Share button, then choose Embed and copy the URL shown below
Then go back to your product’s editing page on Ad-Lister, find the Description box, click on the HTML tab and insert the following code:
<object style="max-width: 100%; width: 475px; height: 260px; float: none; clear: both; margin: 2px auto;" data="the URL you have copied from YouTube, as shown above"></object>
Hit Save at the bottom of the page, and then revise your product with the Description option to see the changes on eBay.