Pass a C#/ ASP variable from to a website

Hello, I'm fairly new to C# and can't seem to find an answer.

I am trying to pass a variable from a C#/ ASP website to an element of a website on a newly opened tab when a button is pressed.

The following code opens the page on a new tab successfully:

protected void SA_Click(object sender, EventArgs e)
    {
        string pageurl = "https://web.nt-ware.net/license/serialinfo.php";
        Response.Write("<script> window.open('" + pageurl + "','_blank'); </script>");
    }

Every answer I've found seems to be the opposite where the user brings data back from a websit.

I know the ID of the element is info_on_srn and I can get this to work in PowerShell using:

$ie.Document.IHTMLDocument3_getElementByID("info_on_srn").value