$(function() {
$('body').append(
$('<input/>')
.attr('type', 'hidden')
.attr('name', 'foo')
.val('some value')
);
});
HiddenField field = new HiddenField();
field.ID = "HiddenField1";
field.Value = "SomeValue";
(this.Page.FindControl("form1") as HtmlForm).Controls.Add(field);