You can easily use Callback Free WordPress Plugin to save yourself the hassle of installing HTML code on your website. Download the archive with the latest version of the WordPress plugin. Log in to the admin panel of your site. In the side menu select “Plugins” -> “Add new” -> “Load new” and load the downloaded archive. Install and activate the Callback Free plugin.
In the WordPress side menu, go to the Callback Free plugin settings and specify the API key. You can find out your API key in your Callback Free account.