Example of adding PostCSS plugins?

Hello. I've tried to get codepan working with PostCSS plugins but I'm not sure how to configure it.

I tried adding a `postcss.config.js` file:

```js
module.exports = {
plugins: {
'autoprefixer': {},
}
}
```
but nothing happens so I think maybe I'm not configuring it correctly?

Do you have an example of adding a plugin?

Many thanks!