default.liquid

<!-- This would go within the {% if sweeps_settings.completed %} section -->
<div id="ref-id" data-id="{{ sweeps_settings.referral_id }}"></div>

gistfile1.js

$(document).on('didRefreshWidgetContent', function () {
  
  var referral_id = $('#ref-id').data('id');
  
  // referral url is canvas_url + reverral_id
  the_referral_url = '';
  the_referral_url = "{{ canvas_url }}" + "?app_" + referral_id; 

});