webpack-dev-server --open to a specific path?

You can also try the before option:

module.exports = {
  //...
  devServer: {
    before: function(app, server) {
      app.get('/', function(req, res) {
        res.redirect('/app'); // here you can try any of the Express JS res options mentioned in the answer below: 
      });
    }
  }
};

Documentation for Express.js response

Good Luck...

v4

Use devServer.open:

npx webpack serve --open /app

or

module.exports = {
  // ...
  devServer: {
    open: ['/app'],
   // ...
  },
}

v3

Use devServer.openPage.

In your case the value would be /app.

You can also rewrite a bit of your config so you don't have to pass cli commands.

output: {
    publicPath: '/app', // deploy on server with /app/ folder name
    filename: '[name].bundle.js',
    path: path.resolve(__dirname, 'public')
},

devServer: {
    contentBase: './dist',
    host: 'localhost',
    port: '3000',
    inline: true,
    compress: true,
    proxy: {
      '/api/**': {
        target: 'http://10.189.1.159:8080',
        secure: false,
        changeOrigin: true,
        cookieDomainRewrite: true
      }
    }
    open: true, // Here
    openPage: '/app' // And here
},