Pass Hidden Source Value with Embed SDK

Hi there, 

I’m trying to pass values from cookies into typeform. I have all the steps working apart from the final one: passing values from a variable into the hidden field within typeform. 

I’m using this very simple code for this:
 

<div data-tf-widget="FY0i2jhu" data-tf-opacity="100" data-tf-inline-on-mobile data-tf-iframe-props="title=Testing HS" data-tf-hidden="hubspot_utk=,hubspot_page_name=,hubspot_page_url=" data-tf-hubspot style="width:100%;height:500px;" id="form"></div>

<!-- https://github.com/js-cookie/js-cookie -->
<!--<script src="https://cdn.jsdelivr.net/npm/js-cookie@2/src/js.cookie.min.js"></script>-->
<script src="//embed.typeform.com/next/embed.js"></script>
<link rel="stylesheet" href="//embed.typeform.com/next/css/widget.css" />

<script>
    var source_var = "testing2"
    window.tf.createWidget('FY0i2jhu', {
    container: document.querySelector('#form'),
        hidden: {utm_source: source_var}
        })
</script>

I have utm source:
 

​​ But I cannot add hidden fields themselves: 

My form is on this page: https://typeform-test-897513.webflow.io/testing-typeform-hs

I’ve also tried adding it here but it doesn’t work: data-tf-hidden="hubspot_utk=,hubspot_page_name=,hubspot_page_url=,utm_source="

How can I pass utm_source value from a cookie?

Thanks