Sharing a video in Webex Webinars is different than sharing a video in Webex Meetings.

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Once in a Webex Event, make sure you are designated as the Presenter. Only the Host can make you the Presenter by right-clicking on your name and selecting Make Presenter, or by dragging the red presenter button to your name on the Participants list. Read more about the Roles in a Webex here.
Click Share at the bottom of your screen.
At the top of the sharing widow, navigate to the File tab and select Share File.
Select an mp4 file from your computer and click Open.

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