Returns the unique code that an inbound remittance quotes so it can be matched to its destination, allocating it on the first call. A destination keeps the same code for its whole lifetime, so the call is idempotent: a repeat request returns the existing code with created: false, and reading a code is just a call that comes back with created: false.

The destination is either a Bakkt account or a remittance beneficiary — send exactly one of account_id or beneficiary_id. Both must belong to the authenticated client.