How to add custom property to ASP.Net TextBox for holding data

There is no way to add custom properties to an ASP.Net Control unless you develop your own custom control.

Other option is to create a UserControl named UCTextBox.ascx and in that add a TextBox

public partial class UCTextBox : System.Web.UI.UserControl
    {
        public string SID { get; set; }

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