Each transfer must have a unique external_reference_id within the bank account.
Attempting to create a transfer with a duplicate external_reference_id will fail with 400 Bad Request.
external_reference_id values will fail.The external_reference_id is unique per bank account, not globally. Different accounts can use the same external_reference_id.
UUID of the service account (e.g., 550e8400-e29b-41d4-a716-446655440000)
Optional idempotency key for safe retries. When provided, requests with the same key return the same result. Without this header, duplicate external_reference_id values will fail with 400 Bad Request.