Special Same Day Return Fares for Octopus Card Users?

I get information from Asia-World expo website that the Holders of valid proof-of-entry of the event are entitled to redeem Same-Day Return Exclusive Fare tickets1 between Hong Kong Station / Kowloon Station and AsiaWorld-Expo Station at just HK$57 with Octopus.
My questions are :

1. Based on information from Asia-World expo website indicate that I have to go to the locate counter to redeem the ticket fare. But from the explanation above that it will rebate automatically once you enter the express on your way back. Could you please advise? And how much is the fare rebate?

2. Do I have to spare enough deposit in Octopus card for round trip before the fare rebate?

Thanks

Best Regards,
Tiana