Function keys default
You can only do that: How to use function keys on MacBook Pro with Touch Bar


  For some apps, you can make the function keys display permanently in
  Touch Bar:
  
  
  In System Preferences, choose Keyboard.
  Click Shortcuts.
  From the left sidebar, select Function Keys.
  Click the “+” symbol, then navigate to the app and select it.
  
  
  Now when you open or switch to this app, Touch Bar always displays the
  function keys.