Hatchbox assigns each application a PORT environment variable to respond to. You'll need to add a process that listens to this PORT.
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
Rack & Sinatra
For Rack & Sinatra apps that don't use Puma, add a rackup process to your app:
bundle exec rackup -p $PORT
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.