On Which Page Event I Can Get Checkbox Checked Value ?

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ASP.NET

cycle

Hi all,
i want to know that on which event i can get that whether check box is chekced or not??

currently i am adding attribute on outside !ispostback to create a javascript event on one button but which works fine but i want to pass checkbox checked value as a query string which is always false even if it is checked.

bool status = chkIsVatApply.Checked;
            string url = "PrintRepairInvoice.aspx?ID=" + Request.QueryString["ID"].ToString() + "&isvat=" + status;
            btnPrint.Attributes.Add("onclick", "window.open('" + url + "', '_newtab');return false;");