For a workaround, can you try using the Backlog project’s webhook with Zapier to send messages to Microsoft Teams?
I think it might be a possible solution. I’ve tried the Backlog webhook with Zapier, and it’s able to send JSON data over to Zapier. And Zapier should in turn be able to send messages to Microsoft Teams.
I wish I could provide a more concrete answer for you. Unfortunately, I do not have a Microsoft Teams paid plan, I’m using the free plan, and it seems that the free plan is not supported for connecting with Zapier.