Sending an Email without submitting form

Hi

I've got no experience with Javascript at all so I'm trying to struggle through this. I've read dozens of posts but they all seem to show how to submit a form, not just send an email (without the form attached) and I don't know how to seperate the two.

Basically, what I want, is a javascript that when they finish the form, they click the submit button and it sends an email to someone in the company letting them know it's complete. The subject of the email needs to be the customer order number (which is one of the fields on the form).

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var subject = this.getField("SalesOrderNumber");

this.mailDoc({
bUI: false,
cTo: "name [at] company [dot] com",
cSubject: subject.value,
cMsg: "The Form is now Complete"
});

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This opens an email with the document attached, the address is correct, the body is correct, but the subject just says: undefined.
I've put this together by piecing little bits from different sites together but it's obviously not right, I just don't know enough to make it right.

Any help?