If using the FPextensions to process the form, you can set the Subject
in Form Properties to use a field (which may be a hidden field) in the
form. Then use JavaScript to concatenate values from other fields
into this; the JavaScript may be triggered by an onclick event on the
Submit button, or by an onsubmit event in the <form> tag (though this
will interfere with any FP validation, if used).
Note that there is a limit to the total length of the field (255
characters), but IMO the subject should be much shorter than that
(unless the email is read by machine rather than people).
