Hi @Simon Bennetts ,
Send Message in Chat Microsoft Graph API currently supports the delegated permissions scope only and hence this Graph API can be used with the signed-in user scope in your app. Since currently Send Message in Chat Microsoft Graph API do not support application permissions scope, the above mentioned requirement is not feasible unless you use the delegated authentication & permissions in your app.
Reference:
https://learn.microsoft.com/en-us/graph/api/chat-post-messages?view=graph-rest-1.0&tabs=http
Hope this helps.
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