Livecycle Designer: Email Subject from Text Field

Problem:
Email Subject from Text Field.

Solution:

1. You need a regular button that calls this script when clicked.  You cannot use built-in Email Submit Button
2. Use can use this script for getting Subject from Text Field

topmostSubform.Page1.Button1::click - (JavaScript, client)

var subject ="Invoice number: " + txtInvoiceNumber.rawValue;

var myDoc = event.target;

try {

    myDoc.mailDoc({

        bUI: false,

        cTo: 'mah@test.ac.uk', // Replace with actual receiver mail address.

        cSubject: subject,

        cSubmitAs: "PDF"

    });

} catch (e) {

    // exception handling...

}