title: How to set ENV var in Heroku preview app postdeploy script
body: <p>I want to set the <code>HOST</code> env var to <code>$HEROKU_APP_NAME.herokuapps.com</code> on a preview app.  It doesn't look like I can do this in <code>app.json</code> since this is a computed value.  </p>
  
  <p>I was hoping to do it in a "postdeploy" script like this</p>
  
  <pre><code>heroku config:set HOST="`heroku config:get HEROKU_APP_NAME -a neon-dev-pr-520`.herokuapps.com"
  </code></pre>
  
  <p>but it wants to authenticate me as a Heroku user.  Alas, this doesn't work either:</p>
  
  <pre><code>export HOST=$HEROKU_APP_NAME.herokuapps.com
  </code></pre>
  
  <p>Any suggestions?</p>