Get the referral code assigned to a customer. Each referral record contains a unique code that the customer can share to invite others to register. When a new customer registers using the code, a customer relation is created between the new customer and the referral code holder.
Default campaign behaviour
When no referral_campaign_id filter is provided, the result contains the code for the project's default referral campaign. To retrieve a code for a specific campaign, pass the referral_campaign_id filter.