TextBox Textbox1 = new TextBox();
TextBox Textbox2 = new TextBox();
    
protected void Button1_Click(object sender, EventArgs e)
    {

        Textbox1.Text = "This is TextBox inside the placeholder";
        Textbox2.Text = "This is TextBox inside the panel";
        Textbox1.Style["width"] = "250PX";
        Textbox2.Style["width"] = "250PX";
     
        PlaceHolder1.Controls.Add(Textbox1);
        Panel1.Controls.Add(Textbox2);
    }