You will not be marked absent . This will happen to your answer sheet  -

There are very less chances that you might forget to write your set code since invigilator always checks all the details before signing the answer sheet . But if it is true that you have forgotten to write the details but in continuation sheet not in the main sheet , then teacher might be kind enough to write the details for you . But if it's in the main sheet , then you need to pray that teacher is good and fill the details by seeing your peers information . Otherwise your answer sheet would be invalid.