1. Go inside the product, go to description tab. Click Tools -> Source code

2. Then copy and paste the code below inside. And click OK

<iframe width="560" height="315" src="/https://www.youtube.com/embed/VIDEO_ID" frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>

Example: youtube.com/watch?v=zV_hRTh9HpE

Copy after ?v= code and paste in Video_ID , after done paste and click ok. 

3. After done add the video, click button save at the top

4. Go to frontend -> Check the video inside the product description