Hi,
Your script is written using Acrobat JavaScript, whereas because you are working in LiveCycle Designer you need to make the script compatible with the LiveCycle Designer form JavaScript.
The code could be changed to this which works in LiveCycle Designer
var cMyMsg = event.target.path
// Assuming the Field_Name field is in the same subform as the button to run the code
var Sub = Field_Name.rawValue;
app.mailMsg({bUI: true, cTo: "someone@somewhere.com", cSubject: Sub, cMsg: cMyMsg});
Hope this helps
Malcolm