Apparently only at Changi Airport or Seletar Airport:
https://www.customs.gov.sg/individuals/going-through-customs/departure/tourist-refund-scheme
That site is light on details. This has a little bit more info. Apparently one requirement is that the items can't be "used or consumed" in Singapore, so it would be ideal if you leave the box sealed just in case they claim you used it. Global Blue does list Apple, but I'm not sure if that includes Singapore. The Singapore IRAS site says that some retailers may participate on their own or with a partner like Global Blue.
https://www.iras.gov.sg/IRASHome/Schemes/GST/Tourist-Refund-Scheme/
https://www.visitsingapore.com/travel-guide-tips/tourist-https://www.globalblue.com/
https://tourego.com/en/retailers
https://www.globalblue.com/
I found this from the Apple Singapore website:
Payment & Security - Shopping Help - Apple (SG)
Can I get GST refund?
GST refund is not available for all purchases (including in-store pick-up orders) made at the Apple Online store. You can shop at an Apple Store to get your GST refund if you are eligible for the tourist refund scheme.