jQuery - Create hidden form element on the fly
Solution 1:
$('<input>').attr('type','hidden').appendTo('form');
To answer your second question:
$('<input>').attr({
type: 'hidden',
id: 'foo',
name: 'bar'
}).appendTo('form');
Solution 2:
$('#myformelement').append('<input type="hidden" name="myfieldname" value="myvalue" />');