Drag a youtube object into your scene from the object panel on the left. You will need to copy the URL of the youtube video you wish to place in your scene, and paste it into the box under the thumbnail in the properties pane.
Since the youtube video is on youtube’s servers and not ours, you will need to click ‘Generate Thumbnail to grab and save a thumbnail of the video. This image will be what your users see in the scene.
Alternately, you can upload a custom image as a thumbnail by clicking the ‘edit’ button on the thumbnail (for example an image with additional text ‘click here to watch on youtube’).