$(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);