This is a Lightstream Labs feature. For info on enabling Labs features please visitthis article

To obtain the information for your scheduled event, you'll want to visit your YouTube Events page and create a "New live event". Once you've created your event, select a stream key type from the following page and you'll see the Ingest Server (Primary Server URL) as well as the Stream Key (Stream Name)

You'll then want to import this information into an RTMP Destination as shown in the gif below:

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