[qualiteam]

You should create a custom module having the following Twig template:

{##
 # @ListChild (list="head", weight="5000")
 #}
<!--  BEGIN chatstack.com Live Chat HTML Code -->
<script type="text/javascript">
	var Chatstack = { server: 'chat.soft-reseller.com' };
	(function(d, undefined) {
		// JavaScript
		Chatstack.e = []; Chatstack.ready = function (c) { Chatstack.e.push(c); }
		var b = d.createElement('script'); b.type = 'text/javascript'; b.async = true;
		b.src = ('https:' == d.location.protocol ? 'https://' : 'http://') + Chatstack.server + '/livehelp/scripts/js.min.js';
		var s = d.getElementsByTagName('script')[0];
		s.parentNode.insertBefore(b, s);
	})(document);
</script>
<!--  END chatstack.com Live Chat HTML Code -->

This should add your custom script into the header part of the page.