Inserting code into html files

I have been hand-writing simple html files for years but am trying to move onto LibreOffice Writer to create and edit html files. My current version is 24.2.7.2 (X86_64).
My web page has an inserted GoogleSheet which is done by the code (characters omitted for readability):
iframe width=“75%” height=“120” src=“https://docs.google.com/document/…?embedded=true” /iframe
(i) I cannot find out how to embed that code using Writer
(ii) When I include that code in a file using Text Editor, the next time I use Writer for that file, it removes that code.
Can I insert such code using Writer, or at least tell Writer not to interfere with certain lines?

Open the plain html in a code editor, insert your code and don’t let LO save it again.