How to use the koa-helmet.hsts function in koa-helmet

if (this.accepts('html', 'text/*', 'text/html')) {
    yield* sendfile.call(this, indexFile)
  }
})

app.use(helmet.csp({
  defaultSrc: ['\'self\'', 'herokuapp.com'],
  scriptSrc: ['\'self\'', '\'unsafe-inline\''],
  styleSrc: ['\'self\'', '\'unsafe-inline\''],
  fontSrc: ['\'self\''],
  imgSrc: ['\'self\'', 'data:']
}))

app.use(helmet.xssFilter())
app.use(helmet.nosniff())
app.use(helmet.hsts({
  maxAge: NINETY_DAYS_IN_MS
}))

// kick off server
if (!module.parent) {
  let server = http.createServer(app.callback())
  mqttBroker.attachHttpServer(server)

  server.listen(config.port, function (err) {
    if (err) {
      return logger.error(err)
    }

    logger.info('app is listening on ' + config.port)
  })
}