On the drop down menu "Options", choose "Email Options.."
Under "Show Message Headers", select the "full" option.
Click the OK button to save the setting.
Juno version 4+ can display MIME and HTML email, but does not provide a way of Viewing the HTML Source for the message within Juno. To get the full source, including HTML codes:
In the Juno mail client, click "file" and then "Save Message as Text File.." (ctrl-T).
Give the file a name which you will remember (many people save temporary files to the desktop).
Double-click on the resulting file and then cut-and-paste the contents to the the form web-site.
After the spam is reported you can delete the temporary file or just over-write it the next time you need to report spam.