esempio:

<form ... action="mailto:...">
<input type="text" name="txt1">
<input type="text" name="txt2">
<input type="text" name="txt3">
<input type="button" onclick="pippo(this.form)">
<textarea name="txtarea"></textarea>
</form>

<script>
function pippo(f) {
f.txtarea.value=f.txt1.value+f.txt2.value+f.txt3.v ale;
f.submit()
}
</script>

ciao