Pass values between popup and home page

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ASP.NET

Controls in the Home page: TextBox, Button
Controls in Pop up: Gridview

What I want to achieve and used so far:
1. Enter text in the textbox and click on the button.

protected void btnLookUp_Click(object sender, EventArgs e)
{
    string sScript = "<script>window.open('a.aspx?textbox=" + TextBox1.Text.ToString() + "','cal','left=10,top=10 resizable= yes scrollbars=yes') </script>";
    RegisterClientScriptBlock("anything", sScript);
}

2. On Button-Click the pop-up opens and on-page load event of the popup page gets triggered.
Gridview control in pop-up page.

<asp:GridView ID="GridView1" runat="server" Font-Size="10px"
    onselectedindexchanged="GridView1_SelectedIndexChanged">
    <Columns>
        <asp:CommandField SelectText="[...]" ShowSelectButton="True" />
    </Columns>
    <AlternatingRowStyle Wrap="False" />
</asp:GridView>

3. On clicking the button for the corresponding entry in the gridview I want to pass selected column values back to the TextBox control in the home page.

protected void GridView1_SelectedIndexChanged(object sender, EventArgs e)
{
    string strScript = "<script>window.opener.document.forms(0)."+ control.Value.ToString() +".value = '";
    //strScript += GridView1.SelectedIndex.ToString();
    strScript += "Amresh';self.close()";
    strScript += "</" + "script>";
    RegisterClientScriptBlock("anything", strScript);
}

The above code gives me error '; missing' as the the column values starts with an integer value.

How do I transfer values back to the textbox in the home from the popup page?
Or can I achieve it using some other method?