This is an old post from 2018 and as Ido has said, you can’t get to the root of your Wix site.
You can however, add your html code from a third party to a new custom tool in Tracking and Analytics that you can put in your head, body or end section.
https://support.wix.com/en/article/embedding-custom-code-to-your-site
https://support.wix.com/en/article/using-iframes-to-display-visible-content-on-your-site
Or, if the third party has a API (preferably REST), then you can expose your Wix site to their API.
https://www.wix.com/corvid/reference/wix-fetch.html
https://support.wix.com/en/article/corvid-accessing-third-party-services-with-the-fetch-api
https://www.wix.com/corvid/reference/wix-http-functions.html
https://support.wix.com/en/article/corvid-exposing-a-site-api-with-http-functions
Example:
https://www.wix.com/corvid/forum/corvid-tips-and-updates/example-myapi-and-myapiclient
Finally, check third parties own websites, like with Subscribers.com…
https://subscribers.freshdesk.com/support/home
It is amazing what you can find there already…
https://subscribers.freshdesk.com/support/solutions/articles/35000119671-wix-installation
Please add a new post if you still require help with this issue