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 **Prettify**.
In **Program** set `prettier`
In **Parameters** set `--write [other opts] $FilePathRelativeToProjectRoot$`
If on the other hand you have `prettier` installed locally, replace the **Program** with `./node_modules/.bin/prettier` (on OS X and Linux) or `.\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.