You can share a YouTube video or a video stored on any server with the file extension mp4 (it is downloaded beforehand, so it takes a moment before it starts playing, therefore the video should be fairly short). On fair.tube it is very straightforward to upload a video and obtain a download link.
Alternatively, you can play a video in a browser tab and share that tab as a screen. Only Chrome / Chromium / Edge browsers are able to also capture the audio from their own tab.