gulp authcode in the main pxt directory will build the authcode project. This is part of the default gulp command, and runs with our build, but you can also run it separately; the command builds authcode and copies the generated index.html file into webap/public as authcode.html, as well as coping all the built JS and CSS files into built/web/authcode.

Run pxt authcode in the target directory to build and serve the authcode project locally. This command copies the appropriate built files (pxtlib.js, target.js, semantic.css, icons.css, react-common-authcode.css) into your local authcode directory to serve, so you may also need to run the appropriate build commands to generate those files.