"Payload" option on Facebook Bots buttons
<p><a href="https://developers.facebook.com/docs/messenger-platform/send-api-reference" rel="noreferrer">Facebook Send API</a> mentions a "payload" type you can set for buttons on the Generic Response Template. However, they don't say how it works, other than:</p>

<blockquote>
  <p>For postback buttons, this data will be sent back to you via webhook</p>
</blockquote>

<p>But how is it sent back? I don't receive any messages when I click the payload button. Has anyone successfully used it?</p>