Here is what I found to get the last ID value and get it to increment.
protected void Page_Load(object sender, Eventargs e)
{
string ID_sel = "SELECT TOP 1 ID FROM table ORDER BY ID DESC";
SqlConnection IDcon = new SqlConnection(Connection string);
SqlCommand cmdsel = new SqlCommand(ID_sel, IDcon);
IDcon.Open();
SqlDataReader read = cmdsel.ExecuteReader();
read.Read();
string a = read["ID"].ToString();
TextboxID.Text = ();
for (int i = 0; i < 1; i++)
{
string val = a.Substring(1, a.Length - 1);
int newnumber = Convert.ToInt32(val) + 1;
a = "B" + newnumber.ToString("000000");
{
TextBoxID.Text = a;
IDcon.Close;