https://secure.na1.echosign.com/public/docs/restapi/v6#!/webhooks/createWebhook

{
  "name": "agreement history",
  "scope": "USER",
  "state": "ACTIVE",
  "webhookSubscriptionEvents": [
    "AGREEMENT_CREATED"
  ],
  "webhookUrlInfo": {
    "url": "MY_SITE_URL_TO_GET_WEBHOOK_EVENT_RESPONSE.php"
  }
}

<?php

$headers =array();
foreach (getallheaders() as $name => $value) {
    $headers[$name] = $value;
}

$myfile = "webhookResponse.txt";
$fh = fopen($myfile, 'a');
fwrite($fh, $_POST."n");
fclose($fh);

http_response_code(200);
return json_encode(["xAdobeSignClientId" => $headers["X-AdobeSign-ClientId"]]);
?>

{
  "code": "INVALID_API_ACCESS_POINT",
  "message": "Request must be made to correct API access point (e.g. use GET /baseUris)."
}