curl --request PUT \
  --url https://restapi.sign.plus/v2/envelope/{envelope_id}/attachments/placeholders \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "placeholders": [
    {
      "recipient_id": "<string>",
      "name": "<string>",
      "required": true,
      "multiple": true,
      "id": "<string>",
      "hint": "<string>"
    }
  ]
}
'

{
  "settings": {
    "visible_to_recipients": true
  },
  "recipients": [
    {
      "recipient_id": "<string>",
      "recipient_name": "<string>",
      "placeholders": [
        {
          "recipient_id": "<string>",
          "id": "<string>",
          "name": "<string>",
          "hint": "<string>",
          "required": true,
          "multiple": true,
          "files": [
            {
              "id": "<string>",
              "name": "<string>",
              "size": 123,
              "mimetype": "<string>"
            }
          ]
        }
      ]
    }
  ]
}