how to create auto increment number in form intextbox

1. public partial class Form1 : Form
{
string str;
SqlConnection con = new SqlConnection();
AutoCompleteStringCollection autoComplete = new
AutoCompleteStringCollection();
// AutoCompleteStringCollection autoComplete = new
AutoCompleteStringCollection();
int incNumber = 0;
private void button3_Click(object sender, EventArgs e)
{
string nyNumber = "s" + incNumber.ToString("00");
incNumber++;
SqlConnection sconn = new SqlConnection(str);
sconn.Open();
SqlCommand cmd = new SqlCommand("insert into Sales_Detail(sd_Id)
values('"+incNumber+"')" ,sconn);
cmd.ExecuteNonQuery();
sconn.Close();
}