You can use variables to fill parameters of the mailDoc method with data from from fields.
var cTo = Textfield1.rawValue,
cSub = "Put in your mail subject here...",
cMsg = "Put in your mail message here...\n\nKind regards\n"; // \n is for line breaks
event.target.mailDoc({
bUI: false,
cTo: cTo,
cSubject: cSub,
cMsg: cMsg,
cSubmitAs: "PDF"
});