Использовать хранимые процедуры.
//Stored Procedure name is ADDCustomer_sp
SqlCommand cmd = new SqlCommand("ADDCustomer_sp", con);
cmd.CommandType = CommandType.StoredProcedure;
SqlParameter Name = new SqlParameter("@CusName", SqlDbType.NVarChar, 50);
Name.Value = TextBox1.Text;
Name.Direction = ParameterDirection.Input;
cmd.Parameters.Add(Name);
cmd.ExecuteNonQuery();