curl --request POST \
  --url https://api.example.com/proposals/{proposal_id}/messages \
  --header 'Content-Type: application/json' \
  --header 'X-API-Key: <api-key>' \
  --data '
{
  "content": "<string>",
  "attachments": [
    {}
  ]
}
'

{
  "detail": [
    {
      "loc": [
        "<string>"
      ],
      "msg": "<string>",
      "type": "<string>"
    }
  ]
}