To add it to the head section, you can create a theme component. Note that the plugin is already going to load the adsbygoogle.js library for you, so once your site is verified by AdSense, you should remove this theme component!
Go to Admin > Customize > Themes > Components and click “Install”. Give it a name like “verify AdSense”.
Then click “Edit CSS/HTML”. In the Common > </head> section, paste the code that AdSense wants to see.
Click Save at the bottom.
Then go to your default theme and add the component.
Now AdSense should be able to find what it wants to see (within 48 hours according to their docs).
Once it shows your site as ready, remove the theme component and fill the plugin’s settings with your publisher ID, ad unit IDs, etc.