Can I specify custom codes or prefixes for the referral codes?

If you would like to use your own codes or specify your own prefixes, you can accomplish this by selecting the "Manually-Added Custom Codes" reward type, then manually adding bulk-created discounts on the Rewards tab of the Campaign Details page. You'll need to keep an eye on how many codes you have left to ensure you don't run out, and periodically add new ones since the app won't make the codes for you automatically in this case.