post https://partners.apploi.com/applicants//message
Creates a new message for a specific application. Messages can be sent to the candidate (outbound) or recorded as responses from the candidate (inbound). Supports both text (SMS) and email message types.
Message Direction:
- 'to_candidate': Outbound messages sent to the candidate (e.g., from AI system)
- 'from_candidate': Inbound messages/responses from the candidate
Email Requirements: When message_type is 'email', the 'subject' field is required.
Authentication: Requires valid x-api-key header.