As of now, Microsoft doesn’t offer a dedicated PowerShell cmdlet to toggle the “Open meeting chat by default” setting for Teams Rooms devices. In other words, the only supported method to change this setting is via the Teams Admin Center (TAC) as you described.
Here are some additional points to consider:
• Although many Teams settings can be managed via PowerShell, this particular option for Teams Rooms hasn’t been exposed through available cmdlets.
• If you’re interested in an automated solution, you might consider providing feedback to the Teams engineering team or monitoring the Microsoft 365 Roadmap and Teams PowerShell module release notes for future enhancements.
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