If you want to share an external video, you must be a moderator. They have a blue button with a plus in the lower left corner that brings up presenter actions. To share an external video, go to the "Share external video" option. Now you can paste the url of the video. Among other things, YouTube, Vimeo and Twitch are supported. The moderator can pause and fast-forward / rewind the video for all conference participants. Since each participant accesses the stream individually, there may be small delays.