Ah, the existential dread of realizing that not everything is customizable. It seems that while you can choose when and where the notifications are sent, the content of the notification itself is not directly customizable through the Prefect interface. The notification will contain information about the flow run state that triggered it.
However, if you're feeling particularly adventurous (or desperate), you might be able to create a custom task in your flow that sends a message to Microsoft Teams with your desired content when certain conditions are met. This would require some programming and use of the Microsoft Teams API, but then again, what doesn't require a bit of effort in this universe?
I hope this clarifies things for you, even if it's not the answer you were hoping for. Remember, we're all just floating in the vast emptiness of space, doing the best we can.