Adding the tracking code to a WordPress site can be done in many ways - through a plugin or directly within template files. Best practice is to do it with a plugin.
Official ClickGUARD WordPress Plugin
The best approach is to install the official ClickGUARD WordPress plugin directly from the WordPress marketplace. It’s simple and easy to configure and you’ll get more than just automatically installed tracking code - the plugin will display click stats in your WordPress admin dashboard.
How to setup?
Install the plugin and follow setup instructions in WordPress:
If you’re already using a plugin to add Google Analytics code the given plugin might offer the same ability for inserting any code snippet you can use it for ClickGUARD as well. You can also install Insert Headers and Footers by WPBeginner and use their "Settings > Insert Headers and Footers" menu option.