Cannot set https proxy in nw.js (former node-webkit)

http://docs.nwjs.io/en/latest/References/App/#appsetproxyconfigconfig-pac_url I learned here that it's possible to set https proxy in nw.js. But my simple script doesn't work. After initializing it I visit a page in my browser but my IP doesn't change. What am I doing wrong?

<script>
var gui = require('nw.gui');
gui.App.setProxyConfig("https=uk.freevpn.pw:443");
</script>

via Igor Zuev