How to send a message to a specific user

narita1234 -
You can send a new chatMessage in the specified chat using this API.
Send message in a chat - Microsoft Graph v1.0 | Microsoft Learn

This API cannot create a new chat; you must use the list chats method to retrieve the ID of an existing chat before creating a chat message.

If you want to create a new chat you can use below API:

 Create chat - Microsoft Graph v1.0 | Microsoft Learn

Thanks, 

Prasad Das

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