It's as easy as
$ ngrok-serve
$ npm install -g ngrok-serve-dir
Usage
$ ngrok-serve <path/to/directory>
$ ngrok-serve <path/to/directory> --port=<PORT>
Options
-p, --port Sets the desired port
Examples
$ ngrok-serve .
$ ngrok-serve ./public/static
$ ngrok-serve ./public/static --port=4000