Get value from dynamically generated textboxes to XML

I added 89 textboxes dynamically. I want to get the value of the textboxes to xml. I can add the textboxes just fine the problem is I cant get the values on those dynamically added textboxes.

For example i want to get value from textBox1 to the node "F1" in XML ,from textBox2 to node "F2" in XML.

private void button1_Click(object sender, EventArgs e)
{  
    XmlNodeList xnList;
    XmlDocument doc = new XmlDocument();
    string dosyayolu = Application.StartupPath + "\\coupling.xml";
    doc.Load(dosyayolu);

    if (globaller.hangimenu == "TWT1")
    {
        xnList = doc.SelectNodes("/coup/TWT1");
    }
    else
    {
        xnList = doc.SelectNodes("/coup/TWT2");
    }

    for (int i = 0; i < 89; i++)
    {
        foreach (XmlNode xn in xnList)
        {
            xn["F" + (i + 1).ToString()].InnerText = "k";
            // xn["F1"].InnerText = textBox1.Text;
        }
    }

    doc.Save(dosyayolu);
}