Normally, you can simply edit the HTML source code and insert extra code. You must know a little about HTML to paste it in the right place. If you don't have that experience, you can copy a backup of your file and then experiment by placing the extra code in various places.
If you use our Sitebuilder, you don't need to guess.
In Sitebuilder, while editing the page, find the icon on the right hand side called Script. Drag and drop that icon into your page where you want the code to appear. Now double-click the Properties link and you can paste any source code you like.