Guest API POST /api/v2/conversations/messaging/conversations returns 400 Bad Request with 'conversationId' missing

POST /api/v2/conversations/messaging/conversations

Payload:

{
 "type": "messaging",
 "to": {
 "id": "123456",
 "type": "user"
 },
 "from": {
 "id": "guest-abc-123",
 "type": "user"
 }
}

Response 400:

{
 "errors": [
 {
 "code": "bad_request",
 "message": "Required field 'conversationId' is missing"
 }
 ]
}

The docs say this endpoint creates the conversation. Why is it asking for an ID that doesn’t exist yet. The guest profile is created separately via the Guest API. Is the sequence wrong or is the endpoint deprecated for direct creation?