How to add additional fields to form before submit?

Question

Yes.You can try with some hidden params.

  $("#form").submit( function(eventObj) {
      $("<input />").attr("type", "hidden")
          .attr("name", "something")
          .attr("value", "something")
          .appendTo("#form");
      return true;
  });

Try this:

$('#form').submit(function(eventObj) {
    $(this).append('<input type="hidden" name="field_name" value="value" /> ');
    return true;
});

$('#form').append('<input type="text" value="'+yourValue+'" />');

You can add a hidden input with whatever value you need to send:

$('#form').submit(function(eventObj) {
    $(this).append('<input type="hidden" name="someName" value="someValue">');
    return true;
});

This works:

var form = $(this).closest('form');

form = form.serializeArray();

form = form.concat([
    {name: "customer_id", value: window.username},
    {name: "post_action", value: "Update Information"}
]);

$.post('/change-user-details', form, function(d) {
    if (d.error) {
        alert("There was a problem updating your user details")
    } 
});