Journal Entry Related Entity on Create

The POST /journal-entries endpoint now accepts an optional related_entity field, allowing you to attach a customer or vendor to a journal entry at creation time.

{
  "related_entity": {
    "id": "<customer-or-vendor-uuid>",
    "type": "CUSTOMER"
  }
}

The type field accepts CUSTOMER or VENDOR. The entity ID is validated against your organization — invalid IDs will return a 400 error. V3 only.