Enviar Documento por URL

<?php

$curl = curl_init();

curl_setopt_array($curl, array(
  CURLOPT_URL => 'https://api.robomensageiro.com.br/api/messages/sendURLDocument',
  CURLOPT_RETURNTRANSFER => true,
  CURLOPT_ENCODING => '',
  CURLOPT_MAXREDIRS => 10,
  CURLOPT_TIMEOUT => 0,
  CURLOPT_FOLLOWLOCATION => true,
  CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
  CURLOPT_CUSTOMREQUEST => 'POST',
  CURLOPT_POSTFIELDS =>'{
    "number": "5511978097649",
    "body": "https://nomedosite.com.br/arquivo.pdf",
    "fileName": "Nome do Arquivo",
    "mimeType": "application/pdf",
    "caption": "Caption"
  }',
  CURLOPT_HTTPHEADER => array(
    'Content-Type: application/json',
    'Authorization: Bearer seu token aqui'
  ),
));

$response = curl_exec($curl);