Hi @Vanessa Li ,
Anyway we can clear that chat history?
I am afraid it's not feasible at present. As a workaround for your scenario, personally I'd recommend setting up a separate webinar beforehand for testing purpose.
Meanwhile, I tried submitting the idea to add a feature to remove chat history of a teams webinar via the dedicated feedback portal for Microsoft Teams. I'll leave the link below so that you and other community can vote or comment there as well:
Delete chat record in teams webinar
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