Seems like you must call to cancel in order to get a full refund. I hope the word must in their conditions below isn’t a deal breaker. I would follow the advice in the post above and hope that does the trick.

https://www.singaporeair.com/en_UK/u...tions-refunds/
Cancellations and refunds
You may cancel your Singapore Airlines ticket online for a refund if you’ve purchased a ticket on a refundable fare. If you cancel a non-refundable ticket, only the taxes will be refunded. The refund will be credited to the credit/debit card used to purchase the ticket. In some instances, a cancellation/refund fee may apply. To qualify for a refund, you must cancel your ticket within the deadline stated on your booking.

If you’ve purchased a ticket for a flight to or from the USA, you may cancel your ticket without penalty within 24 hours of your booking – as long as it’s more than a week before your flight departs. To ensure that you get a full refund, your cancellation must be made with your local Singapore Airlines office.