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////////////////////////////////////////
// Review Pro Popup Formatting
.review-pro-popup-content {
  padding-top: 100%;
  padding-top: 925px;
}
.mfp-iframe-holder .mfp-content {
  max-width: 620px;
}
///////////////////////////////////////

jQuery(document).ready(function($) {
  $('.review-pro-popup').magnificPopup( {
    type: 'iframe',
    iframe: {
      markup: '<div class="mfp-iframe-scaler review-pro-popup-content">'+
      '<div class="mfp-close"></div>'+
      '<iframe class="mfp-iframe" frameborder="0" allowfullscreen></iframe>'+
      '</div>'
    },
  });
});

<a href="http://horizondental.submitreview.co/" class="btn review-pro-popup">Leave us a review</a>