Creates a payee owned by the declared clientId, which must be a client this credential may act for. Bank details are validated against the bank country's configuration and the bank name is resolved automatically. A payee with the same account and bank details as an existing one is rejected with 409. GB sort-code payees require an account name verification first: run POST /account-name-verifications and pass the returned id as accountNameVerificationId; a verification rejected with reason AC01 cannot be used. Optional intermediaryBics carry the BICs of intermediary banks in the payment route: at most 10, each unique, each 8 or 11 characters, and each resolvable to a bank we hold. The 201 body includes accountNameVerification, which is null if the verification service is momentarily unavailable; fetch the payee again later.