Working in Visual Studio

Objective: I am trying to have my server side vb code construct a file name
for use with my client side JavaScript code to then save the file to the
web. My saving to the web method is working with a preset test filename.

The issue I have is that when I put this JavaScript in the source code of an
asp:content holder of a masterpage container, it is not able to see any
server side controls, one of which is returning my filename I want to use.

In a standard HTML page where I worked out the code method it works fine, so
I know the method works.

It is only in asp:content page, where there is no HTML that I cannot read
the server side control.

I can read a HTMLInputText control in the asp:content page so long as it is
not set to runat="server", but then my server side vb code cannot see it to
write my file name to it for use with the JavaScript code.

I either need a way to read the server side control from the asp:content
page with JavaScript,  or write to the HTMLInputText control not set to
runat="server" when the server responds, or a different approach that allows
me to get my filename in a way that the JavaScript can read it.

Thank you for any help,
John

Here is a snippet of the code up to where the error on page occurs:

<%@ Page Language="VB" MasterPageFile="MasterPage.master"
AutoEventWireup="false" MaintainScrollPositionOnPostback ="true"
CodeFile="APcontent.aspx.vb" Inherits="APcontent" title="Accounts Payable"
%>

<asp:Content ID="Content1" ContentPlaceHolderID="ContentPlaceHolder1"
Runat="Server">

<script language="javascript" type="text/javascript">

// <!CDATA[

function File1_onclick() {

  var Var1 = document.getElementById("TextBox1").value; 'ERROR OCCURS HERE

        document.write(Var1);

//File saving code is here.

}

// ]]>

</script>

       <asp:TextBox ID="TextBox1" runat="server"
Width="472px"></asp:TextBox>

</asp:Content>