how does fillup dropdown on page load

protected void ddlSelectSector_SelectedIndexChanged(object sender, EventArgs e)   
      {             int sectorId = SafeConvert.ToInt32(ddlSelectSector.SelectedValue);   
          var empList = _service.GetEmployeeDutyByToSector_Id(sectorId).Where(x => x.EndDate <= DateTime.Today && x.ToSector_Id == sectorId).ToList();  
           ddlRollNo.Bind(empList, "EmployeeIdName", "Employee_Id");               
      }