If you need to add code to the Header you need to use HTML Markup.. menu item in the Page Menu. A window will appear where you can choose where you need the code, as instructed from the site you got it from. Then paste the code into the window.
On this page I have the flavicon code added before the </head> This allows an icon to be added in the browser. I have added tis to the Master Page so that it applies to every page so I do not need to manually add it to every page.