Add Items to ListBox

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C#2.0

C#

ASP.NET

Javascript

Hi Friends,
I want to add items in ListBox on press of (Enter) key in TextBox.
(Just like we do while logging in --put id & password and hit Enter).
I know this is possible with onkeyup event and it worked for me very well.
I have another Button , on click of which I want to get the items in the ListBox.
I managed to do this on client side with no problem.

Now my problem is, when i try to put another item in list box,
I type some text in textbox & hit Enter, the item does not get added in the listbox.
But instead, it calls the button's OnClientClick event.
I wonder how the OnClientClick event is getting fired.

This is my ASPX :

<div>
    <asp:UpdatePanel ID="PnlUpdate" runat="server" UpdateMode="Conditional">
        <ContentTemplate>
            <table>
                <tr>
                    <td align="center">
                        <asp:TextBox ID="txtPONo" runat="server"  onKeyUp="inputKeyUp(event)"></asp:TextBox>
                    </td>
                </tr>
                <tr>
                    <td align="center">
                        <asp:ListBox ID="lstPONo" runat="server" SelectionMode="Multiple" Height="300px"></asp:ListBox>
                    </td>
                </tr>
                <tr>
                    <td>
                        <asp:Button ID="btnAddToList" runat="server" OnClientClick="GetPOListInHiddenField()" Text="Get PO List" />
                    </td>
                </tr>
            </table>
        </ContentTemplate>
        
    </asp:UpdatePanel> 
 </div>

<script type="text/javascript">
    function inputKeyUp(e) {
        e.which = e.which || e.keyCode;
        if (e.which == 13) {
            var varFromBox = document.all('lstPONo');
            var ListBox = document.getElementById('ctl00_MainContentPlaceHolder_lstPONo');
            var TextBox = document.getElementById('ctl00_MainContentPlaceHolder_txtPONo');

            if (document.getElementById('ctl00_MainContentPlaceHolder_txtPONo').value.replace(/\s/g, "") != "") {
                var myOption = new Option();
                myOption.text = document.getElementById('ctl00_MainContentPlaceHolder_txtPONo').value.trim(); //Textbox's value
                myOption.value = document.getElementById('ctl00_MainContentPlaceHolder_txtPONo').value.trim(); //Textbox's value
                ListBox.add(myOption);
                document.getElementById('ctl00_MainContentPlaceHolder_txtPONo').value = '';
            }
            else
                alert("Please enter PO Number");
        }
    }

function GetPOListInHiddenField() {
        var htmlSelect = document.getElementById('ctl00_MainContentPlaceHolder_lstPONo'); //html select box 
        var listLength = htmlSelect.options.length;
        for (var i = 0; i < listLength; i++) {
            document.getElementById('ctl00_MainContentPlaceHolder_HiddenField_PONoList').value = document.getElementById('ctl00_MainContentPlaceHolder_HiddenField_PONoList').value + htmlSelect.options[i].value + ",";
        }
        alert(document.getElementById('ctl00_MainContentPlaceHolder_HiddenField_PONoList').value);
    }

</script>

I am doing this on client side is because, the user will be putting almost over 100 records in List.
And if I go for server side code, that will be costly in performance.

So if you know any better ways,
please suggest me.

Please help me friends.
Thanks,
Lok..