Nothing! It doesn’t do that yet. It should be in the next version - you’ll be able to click on the end point of the route and choose “Add return leg”. 

This is a key feature for me as I catch a train to place and do a route out and back form there.  When will it become available - I looked today and couldn't spot it.  vmt, J

The circular route option is readily available on the website - clear and simple on a computer browser, slightly more complex on a phone browser. 

Actually, the option is available on my Android app. Click on the 'direction' (right turn) arrow on the top left of the screen.