Third-party code injection
Go to the Settings → Code injection
You can inject a third-party code into the head or body line.
Most important things are usually added to the head because information from this element is downloaded first. However, users won't necessarily see it on the screen.
A body will display during the page download in any case, but this element is downloaded after the head.
To learn more on where it is better to inject the code (into the head or body), read recommendations of each particular service. For example, it is better to inject Google Analytics code into the head, since the counter will track the data more correctly.