Arquivo por URL

<?php

$curl = curl_init();

curl_setopt_array($curl, array(
  CURLOPT_URL => 'https://{BACKEND_URL}/api/messages/whatsmeow/sendUrlFilesWhatsmeowPRO',
  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": "554799444499",
    "type": "image", //audio, video, document
    "caption": "",
    "fileName": "algumacoisa.jpeg",
    "body": "https://img-s-msn-com.akamaized.net/tenant/amp/entityid/AA1CrTmb.img?w=612&h=304&q=90&m=6&f=webp&u=t"
  }',
  CURLOPT_HTTPHEADER => array(
    'Content-Type: application/json',
    'Authorization: Bearer seutokenaqui' //Token cadastrado na conexão
  ),
));

$response = curl_exec($curl);

curl_close($curl);
echo $response;