When you open slides manager and select one of the image of the slideshow on the right side of the slides manager you can find additional panel with image configuration settings. One of the options there it’s link field for images. There in options block: Link\Video\JavaScript you can specify video link and configure more flexible with additional parameters below.
If you have problems with video in slideshow try to play with links. For now Youtube videos have alot of different link formats. From time to time Vimeo and Youtube add new short links. My suggestion to use classic video links for your slideshow video.
