I can’t easily answer that question, because it’s different for every theme. This is also why I can’t just automatically configure it. If you can give me a link, I can help more, but without it, all I can say is that you need to find the element that wraps your primary content for the site (most likely something like a <main> element, or a div with ID = main or content). You can do that by viewing source or by using the browser inspector. (Right click, choose inspect element, then navigate within the source shown to find the right container.
Some themes don’t include an ID on the main container, in which case there’s no other choice but to generate a child theme and add the code there.
