Idempotency

Send an Idempotency-Key header on requests that change something, so you can retry safely without doing the same thing twice. The clientRequestId field isn't supported.

How to send it

On POST /api/v1/transactions and other requests that change data, send an Idempotency-Key header of up to 256 characters, using a value you control.

Send the same key with the same body and you'll get the original response back.
Send the same key with a different body and the API returns 409 idempotency_key_reused.
If the first request is still being processed, you'll get 409 idempotency_key_in_progress.

paymentReference is your business reference and does a different job, so please send the header as well. The request body is strict and rejects unknown fields, including clientRequestId.