How to configure Prebid server-side (s2sConfig) to pass keywords to the auction server

Question

Documentation is limited. However, I seek to understand the s2sConfig that will send keywords to the auction server, similar to the prebid.js (pbjs.setConfig) approach - see below:

pbjs.setConfig({
  appnexusAuctionKeywords: {
    genre: ['classical', 'jazz'],
    instrument: 'piano'
  }
});