Creates an instant funding request. The request will be processed and the funds will be
made available to the account in the form of a Memopost non trade activity. Upon settlement
the Memoposted will be corrected to a CSD activity.

Idempotency: When the Idempotency-Key header is supplied, this endpoint is
idempotent. Multiple requests with the same key and identical request body will
create only one transfer. A subsequent request returns the previously created
transfer with the same response (no duplicate is created). If the same key is used
with a different request body, the API returns 422 Unprocessable Entity.

Recommended for production: Always supply Idempotency-Key when creating
transfers. This allows safe retries on timeouts, network errors, or 5xx responses
without risking duplicate transfers. Use a client-generated unique value (e.g. UUID).