customerId property
(optional) id of an off-chain account (managed by the anchor) associated with this user's Stellar account (identified by the JWT's sub field). If the anchor supports SEP-12, the customerId field should match the SEP-12 customer's id. customerId should be passed only when the off-chain id is know to the client, but the relationship between this id and the user's Stellar account is not known to the Anchor.
Implementation
String? customerId;