A few steps involved and your friend needs to have spent at least $20 before 10 Sep, but useful for those who were going to refer their friends anyway.
Full T&Cs
$15 Bonus Promotion Terms and Conditions: 5-10 September 2020
From 05/09/2020 - 10/09/2020 AEST ShopBack rewards you with a $15 bonus for every qualified new member referred. You will earn a TOTAL of $15 bonus ($5 automatic bonus + Extra $10 bonus) for every friend referred successfully. Your friend will need to meet the criteria outlined in Refer-a-friend Programme Term & Conditions and must have submitted their withdrawal details in order for your $5 automatic bonus to be credited. Those referred must make an accumulated min. spend of $20 by 11.59pm AEDT 10 September 2020 for you to qualify for the extra bonus. You will qualify for $10 bonus only when your referral hits the minimum spend of $20 and when their Cashback turns redeemable. The $10 bonus will be credited into your ShopBack account by 22 December 2020. ShopBack reserves the right to terminate this offer at any time without prior notice. Users qualify for this bonus promotion on an individual basis per email account on ShopBack; no accounts can be combined in order to reach the qualified friends requirement.
Couldn't do it in good conscience, even for an extra $10 bribe.
No offence to the OP, but Shopback just have far too many reneged upsize and bonus promos that never get paid. I'll stick with CashRewards.