If you missed the opportunity to add a referral code at sign up, you can still add it before you scan your first receipt. You can enter it by going to the Me tab at the bottom of the app and selecting Refer a friend. On that page, you will be able to enter the code. You are unable to enter it once you submit a receipt, so be sure to get it entered before then!
Even if a link is used to download the app, the referral code may need to be manually entered. If you are unable to enter a code from receiving a referral link, it means the code was automatically entered, and you may scan your first eligible receipt to receive the referral points.
Please note: the referral points will not show on the first receipt itself but will show on the overall account balance on the Activity tab.