Dynamic stamp dialogue not appearing

Acrobat DC Pro on Win 10 Pro.

I have created a dynamic stamp and called the file "Receipt.pdf". I want to use it to signify that my invoice has been paid by my client.

There are two text fields, one above the other, the first to show the current date and the second field I want to generate a dialogue for me to insert particulars of the payment, e.g., a cheque number or other identifier.

When I apply the stamp to the pdf invoice, the date field works fine, but no dialogue is generated. So the stamp appears with the current date and a blank space beneath it.

The "custom calculation script" for the date field is:

event.value = (new Date()).toString();

AFDate_FormatEx("dd mmm yyyy");

The script for the dialogue field is:

var cAsk = "Enter Receipt Details" ;

var cTitle = "Receipt Details" ;

if(event.source.forReal && (event.source.stampName == "#Receipt"))

{

  var cMsg = app.response(cAsk, cTitle);

  event.value = cMsg;

  event.source.source.info.exhibit = cMsg;

}

I am ignorant about javascript and have merely done my own research and followed other examples which seem to work. Would some kind person please help me to get that dialogue field working properly? Any advice will be much appreciated, but please keep it simple!