Entity_picture URL for notifications

Configuration
1

I’m newish to hass but have been playing with it for a bit and am starting to know my way around it.

Right now I’m playing with notifications and have a notification setup to let me know when someone starts playing something on my Apple TV. The notification includes the name of the show using media_title, but I’d also like to play with including attachments in notifications. So I tried adding an attachment and putting the entity_picture property in for the URL.

This has not been working. I am using Node Red and I know that I have the JSON formatted correctly because if I hard code in a URL for a known image the notifications include that image in them as an attachment. However I’m having trouble knowing what to put for the URL so that it grabs the entity_picture.

I’ve tried

{
  "message": "Someone is watching {{states.media_player.living_room.attributes.media_title}}!",
  "title": "Show started",
  "data": {
    "attachment": {
      "url": "{{states.media_player.living_room.attributes.entity_picture}}"
      }
  }
}

and

{
  "message": "Someone is watching {{states.media_player.living_room.attributes.media_title}}!",
  "title": "Show started",
  "data": {
    "attachment": {
      "url": "https://<myurl>{{states.media_player.living_room.attributes.entity_picture}}"
      }
  }
}

the entity_picture property returns

$#x2F;api$#x2F;media_player_proxy$#x2F;media_player.living_room?token$#x3D;<token>

Has anyone done this that can point me in the right direction? Is this even possible?