app.js

$(document).on('didRefreshWidgetContent', function () {
  
  // referral url is canvas_url + reverral_id
  var the_referral_id = $('.ref-id').data('id');

  var the_referral_url = "{{ canvas_url }}" + "?wfrs=" + the_referral_id; 

});

default.liquid

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