Please watch the below video if you'd like to see how to do this (chapter 3).
Navigate to Organisation and Users via the Menu.
Select the Organisation Unit you want to set the default Line Manager against and click the Settings tab.
Click Change to set the Line Manager.
Search for the Line Manager using the people picker.
Click on the radio button next to the name of the Line Manager to select them.
Tick the option to Update all existing users if you you want to apply the default Line Manager to any Users already within the Unit.
The default Line Manger will now be displayed in the Settings tab information.
Any User accounts created in the Unit or moved to the Unit will now be automatically assigned the default Line Manager. In addition, if you ticked the 'Update all existing users' box shown above, any previous Line Manager(s) assigned to the User(s) will be replaced.