update (thanks/credit to ynotrekab):
You’ll need to be a Relish Member to redeem. Sign up below with code ‘MELBOURNE’ and once you’re in, add the Mighty and your burger choice to your order before applying your coupon at checkout.
Got this in the email, valid for grill'd relish members. Expires Thursday and only valid for people in Victoria.
Valid at Participating VIC Grill'd restaurants. Available for Takeaway, online ordering, and Grill’d Delivery. Not valid across Third Party Delivery platforms. Limited to one voucher per customer. Must add chosen main item and Mighty Melbourne to cart before applying coupon at checkout to redeem within online ordering or Grill'd Delivery. Must present and scan Relish promotional barcode via Grill’d email or App to redeem free Mighty Melbourne burger with any main item purchase in restaurant. Additional toppings at extra cost. Single use only. Offer exclusively available to select Relish members. Single-use barcode only. Replica or forged versions of this Relish promotional barcode will not be accepted. Not valid in conjunction with any other offer.