Go to _File | Settings | Tools | External Tools_ for Windows and Linux or _WebStorm | Preferences | Tools | External Tools_ for OS X and click **+** to add a new tool. Let’s name it **Prettier**.
> If on the other hand you have `prettier` installed locally, replace the **Program** with `$ProjectFileDir$/node_modules/.bin/prettier` (on OS X and Linux) or `$ProjectFileDir$\node_modules\.bin\prettier.cmd` (on Windows).
Now when you setup **External Tool** I guess you want to add hotkey for it. Go to _File | Settings | Keymap_ for Windows and Linux _WebStorm | Preferences | Keymap_ and type external tool name in search box.
Go to _File | Settings | Tools | File Watchers_ for Windows and Linux or _WebStorm | Preferences | Tools | File Watchers_ for OS X and click **+** to add a new tool. Let’s name it **Prettier**.