Run npm build after beanstalk deploy

Anyone know how to get node AWS beanstalk to run npm run build command after its does its npm install.

I have the following scripts in my package.json but it doesn't seem to run the build or the start command just the install

  "scripts": {
    "build": "cross-env NODE_ENV=production webpack --config ./webpack/webpack.prod.config.js --progress --colors",
    "start": "node server/app.js",
    "dev": "nodemon server/app.js"
  },

via ericb