how can access to text of a textbox that generated runtime

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C#

ASP.NET

i have an asp.net form and some textbox generated at runtime in Page_Load method

protected void Page_Load(object sender, EventArgs e)
     {
             GenerateControll();
     }

and generatecontoroll() method is

  var n = (from a in db.Parameters
                             where a.fk_OrderTypeID == OrderTypeID
                             orderby a.CodeID
                             select a);
                    foreach (var coll in n)
                    {
          if (coll.fk_ParameterTypeID == 1 || coll.fk_ParameterTypeID == 6)
                        {

                           
                            Label lblKKK = new Label();
                            lblKKK.ID = "lbl" + coll.ParameterID;
                            lblKKK.CssClass = "LabelMargin";
                            lblKKK.Text = coll.ParameterTitle;

                            TextBox txtKooft = new TextBox();
                            txtKooft.ID = "txt" + coll.ParameterID;
                            txtKooft.CssClass = "inputMargin";
                            if (coll.fk_ParameterTypeID == 6)
                            {
                                txtKooft.TextMode = TextBoxMode.MultiLine;
                            }
                            
                            Session.Add(coll.ParameterID.ToString(), txtKooft);

                        
                            pnlForm.Controls.Add(lblKKK);
                            pnlForm.Controls.Add(txtKooft);

                        }
}
}

now my controls generated dynamically, i have a button in may page and in OnClick method of that button i want get the text property of textboxes .
i write below code in my page_Init method

if (Session["19"] != null)
         {
             TextBox t = (TextBox)Session["19"];
             string tID = t.ID;


             string lll = t.Text;
             }

Session[19] is not null and t and tID can find correctly but t.Text is empty while some texts wrote in this Textbox