In the control panel go to the Extensions - Template Manager section.
In the template manager table find the template that is used to render the site. Typically, it has the following attributes:
- System Area - Site
- Display - Yes
Click on the link in the Template column, then on the link Change the template of the main page or select the index.php file. After the <body> tag insert the button code (detailed instructions for obtaining the button code here). Save the template.