I don't think you can insert externa scripts anyway except in a HTML Component. Have you tried that? But then it will reside in the iframes head section and not in the parent site. What is your goal with manychat?
If script is ok to be in HTML component, doesn't need access to parent site structure - you can use HTML comp, and it's actually not WixCode) If it needs access - it's something that you cant achive with WixCode anyway
maybe offtop, but still - i noticed this on your screenshot - "place it into <head> of every page on your site"
This is still not possible with Wix due to many reasons, mainly security... You don't have access to site code "directly"
There's another thread that addresses this. Hoping Wix will engage and help with a solution.