Slack API chat.update returns 'not_authed' error
<p>I have a <em>google script</em> running as a <em>webapp</em> to handle the backend of a <em>slack app</em>.</p>

<p>The app has been <em>Authenticated</em> and I have the OAUTH <strong>token</strong> from this.</p>

<p><em>I can currently post to a channel with button actions using the <strong>chat.postMessage</strong> using the for-mentioned <strong>token</strong>.</em></p>

<p>Actions url points back at my webapp and hook in via doGet, from this response i construct a JSON object.</p>

<pre><code>var response_payload = {
    "token" : access_token,
    "ts" : message_ts,
    "channel" : channel_id,
    "text" : "Approved! you are a winner!"
  })

response_url = "https://slack.com/api/chat.update";

sendToSlack_(response_url, response_payload)
</code></pre>

<p>posted via the following function:</p>

<pre><code>function sendToSlack_(url,payload) {
   var options =  {
    "method" : "post",
    "contentType" : "application/json;charset=iso-8859-1",
    "payload" : JSON.stringify(payload)
  };
  return UrlFetchApp.fetch(url, options)
}
</code></pre>

<p>however returned is the following:</p>

<pre><code>{"ok":false,"error":"not_authed"}
</code></pre>

<p>I can't find any documentation about this error other than the following</p>

<p><a href="https://stackoverflow.com/questions/31905260/sending-json-to-slack-in-a-http-post-request/33310661">Sending JSON to Slack in a HTTP POST request</a></p>

<p>However this is in regard to a chat.postMessage request of which in my implementation is working correctly.</p>