silverstripe-framework/admin/javascript/src/sections/campaign-admin/preview.js

import React from 'react';
import SilverStripeComponent from 'silverstripe-component';

class CampaignPreview extends SilverStripeComponent {

  render() {
    return (
      <div className="pages-preview">
        <iframe src={this.props.previewUrl}></iframe>
      </div>
    );
  }
}

export default CampaignPreview;