how to display textbox on Checkboxlist other click and save into database

 <script type="text/javascript">
        function ChkBoxCheckChange2() {
            var CHK1 = document.getElementById("<%=chkinterest.ClientID%>");
            var checkbox1 = CHK1.getElementsByTagName("input");
            var label = CHK1.getElementsByTagName("label");
            var counter = 0;
            for (var i = 0; i < checkbox1.length; i++) {
                if (checkbox1[i].checked) {
                    if (label[i].innerHTML == "Others, please specify") {
                        document.getElementById("divorgprimary").style.display = (label[i].innerHTML == "Others, please specify") ? "block" : "none";
                    }
                    else {
                        document.getElementById("divorgprimary").style.display = (label[i].innerHTML == "Others, please specify") ? "none" : "none";
                    }
                }
                else {
                    document.getElementById("divorgprimary").style.display = (label[i].innerHTML == "Others, please specify") ? "none" : "none";
                }
                   
            }
        }
        function ValidateModuleList1(source, args) {
            var chkListModules = document.getElementById('<%= chkinterest.ClientID%>');
            var chkListinputs = chkListModules.getElementsByTagName("input");
            for (var i = 0; i < chkListinputs.length; i++) {
                if (chkListinputs[i].checked) {
                    args.IsValid = true;
                    return;
                }
            }
            args.IsValid = false;
        }
    </script>
<div>
        <asp:CheckBoxList ID="chkinterest" runat="server" onclick="ChkBoxCheckChange2(this)"
            RepeatColumns="2" RepeatDirection="Horizontal">
            <asp:ListItem Value="A" Text="A"></asp:ListItem>
            <asp:ListItem Value="B" Text="B"></asp:ListItem>
            <asp:ListItem Value="C" Text="C"></asp:ListItem>
            <asp:ListItem Value="D" Text="D"></asp:ListItem>
            <asp:ListItem Value="Others, please specify">Others, please specify</asp:ListItem>
        </asp:CheckBoxList>
        <asp:CustomValidator runat="server" ID="CustomValidato1" ClientValidationFunction="ValidateModuleList1"
            ErrorMessage="Required!" Display="Dynamic"><span style="color:Red;">Required!</span></asp:CustomValidator>
        <div id="divorgprimary" style='display: none'>
            Others &nbsp;
            <asp:TextBox ID="txtinterest" Width="200px" runat="server" MaxLength="50"></asp:TextBox>
        </div>
    </div>

code----

 string prodInterest = string.Empty;
foreach (ListItem list in chkinterest.Items)
            {
                if (list.Selected)
                {
                    if (prodInterest == "" || prodInterest == null)
                    {
                        if (list.Value == "Others, please specify")
                            prodInterest = txtinterest.Text;
                        else
                            prodInterest = list.Value;
                    }
                    else
                    {
                        if (list.Value == "Others, please specify")
                            prodInterest = prodInterest + "," + txtinterest.Text;
                        else
                            prodInterest = prodInterest + "," + list.Value;
                    }
                }
            }