Requests a new sub-account under the declared parent client, with one default virtual IBAN named by friendlyName and purpose. The request is validated and recorded, then handed to onboarding, which creates the sub-account, reviews it and allocates the IBAN; the response is 202 with the request resource, whose status moves from Pending through Accepted to Active, Rejected or Failed. Follow it with GET /sub-account-requests/{id} or the sub-account-request webhook events. The parent must be a primary client in Client status. A legal entityType requires legalName, jurisdictionOfIncorporationCountryCode and gicsCode; Individual requires givenName, familyName and dateOfBirth. Onboarding rejects a request that matches an existing sub-account of the same parent, in any status: for an individual the same givenName, familyName and dateOfBirth, for a legal entity the same legalName, country and city. The match ignores case and leading or trailing spaces. Retries with the same idempotencyKey and the same details return 200 with the original request. 409 is returned for a key reused with different details, or a byte-identical resubmission under a new key.