Creates a sender registration and sets status to PENDING_APPROVAL.

• If you omit account_id, the API registers the sender for the authenticated account.
• If you provide account_id, the API registers the sender for that account: your own account or one of your child accounts. The API returns 404 Not Found for an account that isn't visible to you, and 403 Forbidden for one that's visible but you don't manage.
• child_account_id is a deprecated name for account_id. If you send both, they must name the same account, or the API returns 400 Bad Request.
• An account you can't use returns 403 or 404. Before these responses were added, a child_account_id you couldn't use returned 400 Bad Request.
• If a registration for the same sender was already created within the last 30 minutes (for the same account or child account), the API returns 429 Too Many Requests. This prevents abuse of the verification retry limit.
• Any existing PENDING_APPROVAL registration for the same sender (for the same account or child account) is automatically cancelled before the new one is created.