Assuming you've already set up an incoming webhook in Microsoft Teams and got the webhook URL, the trick is to get the correct format which MS Teams expects.

The trick is that MS Teams expects a specific JSON structure. This worked for me using custom format:

Format: Custom

Source expression: `{"text":"${_raw}"}`

https://webhook.site/ is an excellent resource for testing what the output of your webhook post looks like (assuming there is nothing sensitive, of course).

Note: Looks like compression needs to be turned off as well.