Returns WMS request.

def _get_wms_request(self, bbox, size_x, size_y):
        """
        Returns WMS request.
        """
        bbox_3857 = transform_bbox(bbox, CRS.POP_WEB)

        return GeopediaWmsRequest(layer=self.layer,
                                  theme=self.theme,
                                  bbox=bbox_3857,
                                  width=size_x,
                                  height=size_y,
                                  image_format=self.image_format,
                                  custom_url_params={CustomUrlParam.TRANSPARENT: True})