Create a sender registration.

Creates a sender registration and sets status to PENDING_APPROVAL.

• If the account does not use parent/child accounts, the API ignores child_account_id and registers the sender for the authenticated account.
• If the account is a parent account and you omit child_account_id, the API registers the sender for the parent account.
• If you provide child_account_id, the API registers the sender for that child account.