pop up windows wherein textbox_onchange doesnt work?

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ASP.NET

Javascript

.NET

hello programmers, got a few errors here that I cannot solve myself.
Ive manage to add a pop up window on my master page so that I could call it everytime

<script type="text/javascript">


        function OpenPopup() {
if (window.showModalDialog) {
    window.showModalDialog("PopUp.aspx", "PopUp",
"dialogWidth:550px;dialogHeight:300px");
} else {
window.open('PopUp.aspx', 'name', 'height=300,width=550,toolbar=no,directories=no,status=no,menubar=no,scrollbars=no,resizable=no ,modal=yes');
}
}
    </script>

See that I am calling PopUp,aspx
Now on my popup.aspx here is the trickest part
I am binding it with autocomplete extender with the source txtSearchBox

<form id="form1"  runat="server">
    <cc1:ToolkitScriptManager ID="ToolkitScriptManager1"  runat="server">
    </cc1:ToolkitScriptManager>

            <asp:Panel ID="Panel1" runat="server">
                <asp:Label ID="lblResult" runat="server" Visible="false" ForeColor="Red" Font-Bold="true"></asp:Label><br />
                <asp:Label ID="Label7" runat="server" Text="Search: " Width="100px"></asp:Label>
                <big><asp:TextBox ID="txtSearchBox"  OnTextChanged="txtSearchBox_TextChanged" AutoPostBack="true" Width="300px"
                    runat="server" Font-Names="verdana,arial,helvetica,sans-serif" Font-Size="XX-Small"></asp:TextBox></big>                <br />
                <br />
                <asp:Label ID="Label1" runat="server" Text="Course Code: " Width="100px"></asp:Label>
                <asp:TextBox ID="txtCourseCode" runat="server" Width="173px" Font-Names="verdana,arial,helvetica,sans-serif"
                    Font-Size="XX-Small"></asp:TextBox>
                <asp:CheckBox ID="checkMultiCode" AutoPostBack="true" Text="*multi codes" runat="server"
                    Font-Names="verdana,arial,helvetica,sans-serif" Font-Size="XX-Small" />
<%--                <div style="removed: absolute; removed 455px; removed 65px; width: 50px">
                    <asp:UpdateProgress ID="UpdateProgress1" runat="server" AssociatedUpdatePanelID="UPanel">
                        <ProgressTemplate>
                            <img alt="Update in Progress" src="Images/load.gif" height="30px" />
                        </ProgressTemplate>
                    </asp:UpdateProgress>
                </div>--%>
                <br />
                <br />
                <asp:Label ID="Label2" runat="server" Text="Course Name: " Width="100px"></asp:Label>
                <asp:TextBox ID="txtCourseName" runat="server" Width="250px" Font-Names="verdana,arial,helvetica,sans-serif"
                    Font-Size="XX-Small"></asp:TextBox>
                <%--<asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server" ControlToValidate="txtCourseName"
                    Font-Names="verdana" Font-Size="XX-Small" ErrorMessage="Needed"></asp:RequiredFieldValidator>--%>
                <br />
                <br />
                <asp:Label ID="Label3" runat="server" Text="Duration: " Width="100px"></asp:Label>
                <asp:TextBox ID="txtCourseDuration" runat="server" Width="250px" Font-Names="verdana,arial,helvetica,sans-serif"
                    Font-Size="XX-Small"></asp:TextBox>
                <%--<asp:RequiredFieldValidator ID="RequiredFieldValidator2" runat="server" ControlToValidate="txtCourseDuration"
                    Font-Names="verdana" Font-Size="XX-Small" ErrorMessage="Needed"></asp:RequiredFieldValidator>--%>
                <asp:RegularExpressionValidator ID="RegularExpressionValidator1" runat="server" ControlToValidate="txtCourseDuration"
                    Font-Names="verdana" Font-Size="XX-Small" ErrorMessage="Numbers/Decimal Only"
                    ValidationExpression="(^\d*\.?\d*[1-9]+\d*$)|(^[1-9]+\d*\.\d*$)"></asp:RegularExpressionValidator>
                <br />
                <br />
                <br />
<%--                <div style="display:none;">
                <asp:Button ID="button2" runat="server" HorizontalAlign="right" Width="200px"
                    Text="Update" align="right"/>
                    </div>--%>
                <asp:Button ID="ButtonUpdateCategory" runat="server" HorizontalAlign="right" Width="200px"
                    Text="Update" Visible="false" align="right" OnClientClick="CloseDialog()" />
                <asp:Button ID="ButtonSave" runat="server" HorizontalAlign="right" Width="200px"
                    Text="Save" align="right" />
                <asp:Button ID="btnCancel" runat="server" Width="200px" CausesValidation="false"
                    align="right" Text="Cancel" />
                <asp:Button ID="Button2" runat="server" align="right" Text="Cancel" Style="display: None" />
                <br />
                <br />
                <asp:TextBox ID="txtCourseCatID" Style="display: none" runat="server"></asp:TextBox>
                <asp:TextBox ID="txtCourseID" Style="display: none" runat="server"></asp:TextBox>
                <asp:TextBox ID="txtCode" Style="display: none" runat="server"></asp:TextBox>
                <div id="div1">
                    <cc1:AutoCompleteExtender ID="AutoCompleteExtender2"  runat="server" TargetControlID="txtSearchBox"
                        CompletionListCssClass="AutoExtenderCourse" CompletionListItemCssClass="AutoExtenderList"
                        CompletionListHighlightedItemCssClass="AutoExtenderHighlight" CompletionListElementID="div1"
                        CompletionSetCount="10" ServicePath="WebService.asmx" ServiceMethod="CourseCode"
                        CompletionInterval="10" MinimumPrefixLength="1">
                    </cc1:AutoCompleteExtender>
                </div>
            </asp:Panel>

    </form>

See that my txtSearchbox has ontextchange and auto postback. Now i can get the value i want but then it doesnt remain in the window popup thus it transfer the value to a new window.

Protected Sub txtSearchBox_TextChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles txtSearchBox.TextChanged
        txtCourseName.text = txtSearchBox.text    End Sub