In the project directory, run the Tabris CLI command:
tabris serve
This will start a JavaScript app code server at a free port and print its URL to the console.
The JavaScript app code can be side-loaded in the developer app if the default config.xml was not changed. Otherwise, the JavaScript app code must be side-loaded in a debug build of this app.
The app can be built using the online build service at tabrisjs.com or locally using Tabris.js CLI.
See Building a Tabris.js App for more information.