- Build the package with yarn build:package
- Go on Admin -> Apps & Integration -> Apps -> Available Tab -> Upload App
- Select your package, which should be inside ./build
- Click the Scratchpad App, go on Setup and click Install at the bottom
- For development purposes, you should include the URL where you're running the app. For this, click on Scratchpad App inside the Installed tab, click on developers, toggle on Development mode and Insert the URL, and press Save at the bottom.