How to Auto populate text boxes when selecting value from Dropdownlist

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C#

ASP.NET

following code is for load dropdown list from database. But i want to show a selected data from dropdownlist in textbox with all deails.

protected void load_customer()
{
   using (MySqlConnection con = new MySqlConnection(constr))
   {
      using (MySqlCommand cmd = new MySqlCommand("SELECT concat(firstname, ' ', lastname) as FullName, CustNo FROM tblcustomer order by CustNo"))
      {
         cmd.CommandType = CommandType.Text;
         cmd.Connection = con;
         con.Open();
         ddCustomer.DataSource = cmd.ExecuteReader();
         ddCustomer.DataTextField = "FullName";
         ddCustomer.DataValueField = "CustNo";
         ddCustomer.DataBind();
         con.Close();
      }
   }
   ddCustomer.Items.Insert(0, new ListItem("--Select Customer--", "0"));
}