How to set the default button for a TextBox in ASP.Net?

Question

I have a page that has two buttons, btnSearch and btnAddUser. The aspx page is as below. As you can see the default button is btnAddUser. But I would like to set the default button to btnSearch when I type something in the text box txtFilter. I added a JavaScript function clickButton, and in the page_load I added following code. The problem is when I press enter after I type something in the text box, both btnSearch and btnAddUser are clicked. Is there any way I could do so that btnAddUser is not clicked.

 txtFilter.Attributes.Add("onkeypress", "return clickButton(event, '" + btnSearch.ClientID + "')");

<asp:Panel ID="MainPanel" runat="server" DefaultButton="btnAddUser">
    <table align="center" width="900">
        <tr>
            <td align="left" width="70%">
                <asp:TextBox ID="txtFilter" runat="server" Width="200"></asp:TextBox>
                <asp:Button ID="btnSearch" runat="server" Text="Search" 
                    onclick="btnSearch_Click" CausesValidation="false" />
            </td>
            <td align="right" width="30%">
                <asp:Button ID="btnAddUser" runat="server" Text="Add User" Width="85px" 
                    CausesValidation="False" onclick="btnAddUser_Click" />
            </td>
        </tr>
        <tr>
...
</asp:Panel>

function clickButton(e, buttonid) {
            var evt = e ? e : window.event;
            var bt = document.getElementById(buttonid);
            if (bt) {
                if (evt.keyCode == 13) {
                    bt.click();
                    return false;
                }
            }
        }

UPDATE: After googling, i found a solution as below. It seems working. But it is not working on Firefox? Anyone know how to resolve it?

function clickButton(e, buttonid) {
            var evt = e ? e : window.event;
            var keycode = evt.keyCode || evt.which || evt.charCode;
            var bt = document.getElementById(buttonid);
            if (bt) {
                if (keycode == 13) {
                    evt.cancelBubble = true;
                    evt.returnValue = false;
                    if (evt.stopPropagation) {
                        evt.stopPropagation();
                        evt.preventDefault();
                    }
                    bt.click();
                    return false;
                }
            }
        }

Answer 1

You don't need javascript just wrap search text box in another panel and set default button, something like this (i stripped table tags) :

  <asp:Panel ID="MainPanel" runat="server" DefaultButton="btnAddUser">
    <asp:Panel ID="searchPanel" runat="server" DefaultButton="btnSearch">
      <asp:TextBox ID="txtFilter" runat="server" Width="200"></asp:TextBox>
      <asp:Button ID="btnSearch" runat="server" Text="Search" 
        CausesValidation="false" onclick="btnSearch_Click" />
    </asp:Panel>
    <asp:Button ID="btnAddUser" runat="server" Text="Add User" Width="85px" CausesValidation="False" onclick="btnAddUser_Click" />
  </asp:Panel>