Use of Entity selector ' supported_features'

I see this in the Docs but have not cracked how to format the keyword into the select statement. I am building something that will mute a media device, and want to use the supported_features to only give me those that will do #8 allow mute. Anytime the keyword is in there is does a syntax error and the script does not render into the yaml base. Doesn’t matter if I use a number or something else.
I thought it should be on the same level as domain, etc…
Now I know that code server doesn’t like it, but beyond that, the script will not reload if this line is in there, either. Is this actually part of the schema? Comment that line and it reloads fine, so syntax or, something…

    fields:
      entities:
        name: Media Player 2 mute - unmute
        description: Set the Google media player you want to mute
        required: true
        selector:
          entity:
            multiple: true
            domain: media_player
            supported_features: 8