Unable to make a direct POST webhook call

Summary: Im trying to make a stand alone POST call to my ODA webhook URL

Content (required): I've enabled the channel, I've generated the authentication code, but i am always receiving the 403 forbidden error

Version (include the version you are using, if applicable): 21.12

Code Snippet (add any code snippets that support your topic, if applicable):

var crypto = require('crypto');

const body = Buffer.from(JSON.stringify('{"userId": "398383","messagePayload":{"type": "text","text": "hello, world!" }}'), 'utf8');

  const headers = {};

  headers['Content-Type'] = 'application/json; charset=utf-8';

  var cgec = buildSignatureHeader(body, "test");

  console.log(cgec);

function buildSignatureHeader(buf, channelSecretKey) {

  return 'sha256=' + buildSignature(buf, channelSecretKey);