Create a basic Wix App that will append a given script just before the closing </body> tag on your Wix Website.
This will be an app that can be found and downloaded in the Wix App Store that any user can install, enter a script, enable it and save it. When they hit save it will then append to the body of each page on the site.
Attached is an app that we already created on Wordpress to do this, and will follow the same format but for Wix.