Copy the script text provided by the third-party service (highlight and either press CTRL + C, or right-click and select Copy).
Log in to your Admin Area.
Go to Marketing > SEO.
With the Default Values tab selected, paste the the text into the Globally Appended Meta Tags field (either press CTRL + V, or right-click and select Paste).
If you need to install an ROI tracking code as well, see How to Install ROI Tracking Conversion Code on Your Site.