Check your emails. I almost missed this offer before booking a flight last night!
Also, spend responsibly!
Save the offer to your eligible Card and spend $350 or more, in one or more transactions, on flights within or departing Australia at qantas.com by 17/09/2019 to receive one $100 credit. Limited to first 10,000 Cards to save the offer. Exclusions apply.
To check if you're eligible, go here: https://global.americanexpress.com/offers/eligible
What is the URL to browse to offers available to add to your card? I can't seem to find it in my AMEX portal
edit: found it - https://global.americanexpress.com/offers/eligible