AJAX :: Running Client Script To Read TextBox Value From PopupControl Extender

I used PopupControl Extender to display row details from a gridview, and need to update the two textboxes, but the changed value in the textboxes did not get picked up when click the update button on the popup window, so I need to get client side javascript to read the new changed value from these text boxes. However, I tried a few way, none worked for me, not sure what is wrong.

1. Add a javascript function in the .aspx file and use onclientclick of the linkbutton control to call the function, I got error of "Microsoft JScript runtime error: Object required".

<script language="javascript" type="text/javascript">
function newTextValue(textbox)
{
var text = document.getElementById("TextBox1").Value;
alert(text);
}
</script)
<asp:TextBox ID="TextBox1" runat="server" Text='<%# Bind("GalleryTemplateName") %>' Height="20px" Width="230px"
style="text-align: left"></asp:TextBox>
<asp:LinkButton ID="LinkButton2" runat="server" Font-Bold="True" CommandArgument='<%# Eval("GalleryTemplateId") %>'
oncommand="LinkButton2_Click" OnClientClick="newTextValue(this)" >Update</asp:LinkButton>
2. Use ScriptManager.RegisterClientScriptBlock in the aspx.cs file, :
ScriptManager.RegisterClientScriptBlock(this, this.GetType(), "changevalue", "alert(document.getElementById('TextBox1').value);", true);

but the alert window did not showup, that means the client javascript did not run? I just use an alert window to test to see if the textbox value is picked up. Note, I used Ajax Toolkit PopupControl Extender, all codes are in an update panel.