Add any email that is active and you can get the info from it. Personally I would suggest that you should add your email/your parents email where the payment receipt and all those stuff can be stored safely. The purpose of this is to just keep information from the Gateway through which you are paying that the payment is done and a specified number will be provided namely 'Transaction ID'.

In future if you face any problem you can use it as a proof beside your bank money deduction message.

I hope my answer helps. All the very best for your future endeavors!