For Windows click on 'File' -> Preferences -> Settings. Then search for editor.formatOnSave, and put "true" instead of "false"

For Mac click on 'Code' (top left of screen, to the left of 'File', 'Edit', and 'View') -> Preferences -> Settings -> enter the term 'editor.formatOnSave' in the search settings box at the top of the page. Click on the tick to mark it as true.