I want to add items to an ASP.Net combobox using Javascript

//Fill the years (counting 100 from the first)
    function fillvarYear() {
        var dt = $('#txtBDate').val();
        dt = dt.toString().substring(6);

        var ye = parseInt(dt);

        //Loop and add the next 100 years from the birth year to the combo
        for (var j = 1; j <= 100; j++) {
            ye += 1; //Add one year to the year count

            var opt = document.createElement("OPTION");
            opt.text = ye;
            opt.value = ye;
            document.form1.ddlYear.add(opt);
        }
    }

string selectedValue = Request.Params[combobox.UniqueId]