I called WIX service which told me that they do support the access to third party web services (https://support.wix.com/en/article/guidelines-and-limitations-of-the-html-code-and-embed-a-site-elements).
I tried the 2 ways to do it (through the "Html" widget, and through "Tracking & Analytics" of the editor), but it is still not working.
I will copy-paste the code below (to get referrals, which is VERY important for my product)...Thanks !
<script>window.jQuery || document.write('<scr' + 'ipt src="https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.0/jquery.min.js"><\/sc' + 'ript>')</script>
<script>window.$kol_jquery = window.jQuery</script>