Before your webinar begins, while you are in the Green Room, your webinar is set by default to start Recording when you Go On Air:

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You may decide not to Record immediately for example if you want to make an announcement to your audience before starting or if a Guest Presenter is running late.  You may then uncheck the box shown above and manually start your Recording in your webinar.

At the top of your screen, simply press the Record icon:

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The icon will show as red when Recording. Press it again at any time to stop Recording:

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If you wish to restart Recording, you are offered the choice to Append that recording (continue where you left off) or to Overwrite the Recording.

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