$(document).ready(function () {
    $("input#ctl00_ContentPlaceHolder1_bkmain1_bkprehudinfo1_ckcopyaddress").bind("click", function (o) {
        if ($("input#ctl00_ContentPlaceHolder1_bkmain1_bkprehudinfo1_ckcopyaddress:checked").length) {
            $("#ctl00_ContentPlaceHolder1_bkmain1_bkprehudinfo1_txtG1").val($("#ctl00_ContentPlaceHolder1_bkmain1_bkprehudinfo1_txtD1").val());
        } else {
            $("#ctl00_ContentPlaceHolder1_bkmain1_bkprehudinfo1_txtG1").val("");
        }
    });
}
    );