NSExtensionServiceTouchBarIconName
The name of an icon for display when the extension appears as a Quick Action in the Touch Bar
Discussion
This key is used in conjunction with the NSExtensionServiceAllowsTouchBarItem key.
Set the NSExtensionServiceTouchBarIconName key’s value to a system icon name or the name of an icon within the extension bundle. This icon should be a template image: a monochromatic image with transparency, anti-aliasing, and no drop shadow that uses a mask to define its shape. For design guidance, see Human Interface Guidelines > macOS > Custom Icons. If no icon is specified, a default icon is used.