L.TileLayer.WMS_Headers = L.TileLayer.WMS.extend({
  createTile(coords, done) {
    const url = this.getTileUrl(coords);
    const img = document.createElement('img');
    superagent
      .get(url)
      .set(‘header’, ‘header value’)
      .responseType('blob')
      .then((response) => {
        img.src = URL.createObjectURL(response.body);
        done(null, img);
      });
    return img;
  }
});

L.tileLayer.wms_headers = (url, options) => new L.TileLayer.WMS_Headers(url, options);