Techincally that is incorrect. When the current line ends is called a linebreak, not a "CR-LF sequence". In HTML a CR-LF pair will be ignored or treated as space character instead of causing a linebreak, except in certain elements or conditions.
One thing that you could do is to use a web page with a copy of a WYSIYG HTML editor. Visit http://www.htmlarea.com/ - AreaEdit and click on "Demo". Type some text in, then find the button "< >" and when you click it, it will toggle to show you the HTML source. You can then copy and paste that into the web panel for the message.
In addition, if you must send the message as text/html, but you want to suggest the mail client display with a monospace font and interpret CRLF as a linebreak, you can enclose the content in