Hi Marcy,
Currently we only read the video ID from video links, so no URL parameters are supported. Since this is the only way YouTube supports looping you can't do that at the moment.
We will be making more configuration options available for videos in the future. We want to give you controls in Builder to enable features like looping or starting the video at a specific time rather than having to know the URL parameters.
Thanks again for your feedback!
Owen
Owen Evans
Lead Product Engineer | StoryMaps