Because SiteSpinner is more of an HTML (or SVG) generator rather than an HTML editor, there is no HTML code stored that can be edited to affect the page display. That means the code is generated every time you preview.
What you can do, however, is insert code of any kind as a 'Code Object'. The code can then be placed anywhere in the document. The text of a Code Object will be added into the HTML page at the position (front to back) of that object with respect to the other objects on the page.
See here for more information on working with code.