Navigate to the Team you want to add the Reflect app to.

Once inside the Team channel, click the (+) icon at the top of the window to add more apps and materials. 

The Reflect app likely won't appear, so click "More apps" to launch the Apps integration store. 

When you click it, a pop-up window should appear. Beside the "Open" button is a dropdown arrow. 

Click the dropdown and choose "Add to a team" to add the app into a specific class/course. 

This will add the Reflect app to your team. From there, click "Create a new Reflect check-in" to add a reflection to the Team's channel. 

How to add "Reflect" check-ins to Microsoft Teams