setManifestKeyPrefix(manifestKeyPrefix) {
        webpackConfig.setManifestKeyPrefix(manifestKeyPrefix);

        return this;
    }
Used as a prefix to the *keys* in manifest.json. Not usually needed.

You don't normally need to set this. When you *do* need to set
it, an error will notify you.

Typically, publicPath is used in the keys inside manifest.json.
But if publicPath is absolute, then we require you to set this.
For example:

```
Encore
    .setOutputPath('web/build')
    .setPublicPath('https://coolcdn.com/FOO')
    .setManifestKeyPrefix('build/')
```

The manifest.json file would look something like this:

```
{
    "build/main.js": "https://coolcdn.com/FOO/main.a54f3ccd2.js"
}
```

@param {string} manifestKeyPrefix
@returns {Encore}