'klayvioForms' event not being fired on form submission

Hello. I’m attempting to submit an event to our mixpanel dashboard when a user submits their email address to a signup form. I have this code snippet before the closing </body>

<script>
  window.addEventListener("klaviyoForms", function(e) {
        if (e.detail.type == 'submit') {
          let formLocation = 'Popup';
          if (e.detail.formId === '<FORM_ID>' || e.detail.formId === '<OTHER_FORM_ID>') {
            formLocation = 'Footer;
          };
          mixpanel.track('Email List Signup', {
            'Form location': formLocation,
            'User email': e.detail.metaData.$email 
          });
        };
    });
</script>

I added a console log statement in there as well but was not seeing it get hit when I submit either of the forms. It’s like the listener for `klaviyoForms` is failing. Any help here would be great, 

Thank you