Coba hapus ' dari kueri Anda dan gunakan Prepare setelah menambahkan parameter:
cmd = new MySqlCommand("SELECT * FROM admin WHERE example@sqldat.com AND admin_password=PASSWORD(@val2)", MySqlConn.conn);
cmd.Parameters.AddWithValue("@val1", tboxUserName.Text);
cmd.Parameters.AddWithValue("@val2", tboxPassword.Text);
cmd.Prepare();