I'm guessing here, because this is not something I use. I've found an example in a book which has
<form action="mailto:myemail? subject="test" method="post" enctype="text/plain">
so it looks as if you were right there. I can't find an example of onsubmit, but I still think you'd need the closing double-quote there. Try it and see if it works, and I'll keep reading the book.
Edit: Actually, a quick search suggests it should be onSubmit, so try that too.