If you would like to learn how to add widgets, you can visit this article.
Drag the Youtube Widget to where you want it to appear inside the Scene.
Copy the video id (it is the part after v= on the youtube link).
In the right-hand side menu, paste the URL of the Youtube video you want to add into the Edit value section.
Customize the size and other features of the widget to adjust how the video will appear inside the Scene.
Save your changes and publish your content.