There are two ways to add a new stop between existing points on an itinerary.
In the right hand list, hover over the map point which will be before your added location. Click the “edit” button (shaped like a pencil) and select “Add next destination” from the list (note that clicking it will not make the list disappear). Move to the map and select the location you would like to add. Complete the name and method of transport and click ‘Save’. The place will be added to the map and the itinerary and the map will automatically reroute the line.
Alternatively, you can right click on the point before where you want to insert the new point. Select ‘Add Next Destination’ feature, and then click on the map to select the new point.
Click save and the map will re-route the line.