Welcome to TalkGraphics David
As Boy points out, Web Designer generates the HTML code and associated files when you publish. So even if you edit the HTML document in a text editor, Web Designer will overwrite your changes the next time you make a change and publish your site.
If you need to add a script or line of code to your page can do so using a Placeholder object, which can be anything, usually a simple rectangle.
Select the placeholder object, go to Website Properties > Placeholder and then click the Insert HTML Body or Head button and paste your text or script. Placeholders are invisible when you publish but insert your code or script into the page or site every time you publish.
For more information see Help > Index > Placeholder Objects
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