When signing up for the application, a prompt will appear asking if you'd like to enter a referral code:
Enter_a_Referral_Code_
If you skipped this chance, you can enter a referral code later on by tapping on the Me tab and selecting Refer a friend. It is important to note that you must enter the referral code before the first receipt is snapped or scanned. If the first receipt is already snapped or scanned, a referral code is no longer able to be entered