This is a Lightstream Labs feature. For info on enabling Labs features please visit this article
All information in this guide is to be used for setting up Prism by Lightstream, available at https://prism.golightstream.com/#/.
To add Prism as a destination inside of Lightstream, you'll want to first start a project on Prism. Once created, you'll see the necessary RTMP information inside of the position you'd like to occupy on stream.
You'll then want to import this information into an RTMP Destination as shown in the gif below: