Requests a virtual account for a participant. The account is created asynchronously: this endpoint returns 202 Accepted with status: PENDING and no bank_details, and the account becomes ACTIVE once the banking partner issues it. Poll GET /virtual_accounts/{id} for the transition. Idempotency is keyed solely on the required Idempotency-Key header, which also becomes the account id — replaying the same key returns 200 OK with the existing record, while a different key against the same participant creates a second, distinct account.