Hi @Gene-Enli (蔡承祐)
Sending chat messages in an application-only context is not supported, and application permissions can only be used to import third-party platform messages.
To send chat messages, you should use delegated permissions with user participation.
Code snippet you can follow: https://learn.microsoft.com/en-us/graph/sdks/choose-authentication-providers?tabs=csharp#authorization-code-provider.
Hope this helps.
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