How to put value in asp.net textbox placed in content page using JavaScript? (ASP.NET, C#, JavaScript)

When using master page and content page in asp.net, original ids of the controls (used in aspx page) are changed during execution of the page, because the whole page will be served as HTML page to the viewer. The ids are prefixed with ContentPlaceHolder ID and some other information.

So what we can do is, we can match the original ids of the control, within this new ids generated by asp.net server to find that control.

Once we get the control, we can do our normal java scripting.

Example : A search page opens a new window upon clicking on "search" button. This new window passes some value to search page's textbox and closes itself upon clicking on its "Click" button. Both pages uses a master page.

For example I have a master page and 2 content pages(page1.aspx, page2.aspx).

Page1.aspx have following code:

<asp:Content ID="Content1" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">
<script type="text/javascript">
function OpenPopup()
{
    window.open("Default2.aspx","List","scrollbars=yes,resizable=yes,width=550,height=480");
    return false;
}
</script>

<asp:TextBox ID="tbx_Name" runat="server"></asp:TextBox>
<asp:Button ID="bt_search" runat="server" Text="search" />

</asp:Content>

Page1.aspx.cs page have following code:

protected void Page_Load(object sender, EventArgs e)
{
     bt_search.Attributes.Add("onclick", "javascript:return OpenPopup()");
}

Page2.aspx have following code:

<asp:Content ID="Content1" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">
<script language="javascript">
function GetUserValue(val)
{
       frm=window.opener.document.forms[0];
       for (var i=0;i<frm.length;i++)
      {
            str=frm.elements[i].id;
            if (str.search("tbx_Name")>=0)
               { frm.elements[i].value="testing...";
               }
        }
      window.close();
}
</script>

<asp:Button ID="bt_UserName" runat="server" Text="Click" OnClick="bt_Click" />
<asp:Button ID="bt_Close" runat="server" Text="Close" />

</asp:Content>

Page2.aspx.cs page have following code:

protected void Page_Load(object sender, EventArgs e)
{
      bt_UserName.Attributes.Add("onclick", "javascript:GetUserValue('" + "Tim Johnson" + "')")
}

protected void bt_Click(object sender, EventArgs e)
{
      if (!Page.IsClientScriptBlockRegistered("ClosePopUp")) 
      {
              RegisterStartupScript("ClosePopUp", "<script language=\"javascript\">function CloseMe()      {window.close();window.history.back(); };</script>");
      }

      bt_Close.Attributes.Add("onclick", "ClosePopUp()");
}