How to append POST parameters to submitted form?

I'm new to LiveCycle and need help.

I want to create a form in Designer with a submit button that submits the form data back to my Servlet.  I see an option on the Designer to enter my submit URL, but where can I specify a  parameter to set before the submission?  For example, before the submit is processed, I want to set a parameter called "mode" with a value of "pdftest".

If I was doing this on a JSP page, I would just add an onClick javascript event to the button and in the javascript method, I would do something like this:

  function processAdobe(theForm) {

        theForm['mode'].value = "pdftest";

        theForm.submit();

   }

Then in my servlet, I could extract this with a call to request.getParameter("mode");

What is the equivalent for setting this up in my .xdp form in Designer?