Qs

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/qs/6.5.2/qs.js"></script>
<pre id="output"></pre>

JavaScript

const $pre = document.querySelector('#output');

const queryString = Qs.stringify({
	crds: [
  	{
    	crd: 1,
			participantIds: [1,2,3]
    }
	]
});

$pre.innerHtml = queryString;