Create mail scripts in System Notifications > Email > Notification Email Script, and refer to them by using ${mail_script:script name} in the script field.

To print text into the body of the message, use the template.print("a string") function.

Figure 1. JavaScript in templates

The event.parm1 and event.parm2 parameters that come from the originating event can also be used.

For additional information on creating mail scripts, see Notification Email Scripts on the Developer Site.