Hatchbox assigns each application a PORT environment variable to respond to. You'll need to add a process that listens to this PORT.

Puma
If your app has Puma in the Gemfile, Hatchbox will automatically detect it and add a Process to run Puma which will start the application defined in config.ru.

Rack & Sinatra
For Rack & Sinatra apps that don't use Puma, add a rackup process to your app:

bundle exec rackup -p $PORT 

Hanami
Add a Hanami server process to your app:

bundle exec hanami server -p $PORT

Other Ruby Frameworks
For other Ruby frameworks, consult their documentation to determine how to run your application's server on a specific port.