Using friendly_name as string in json

I have my esphome code setup to send a pushover message after boot. it works fine except I’d like to use the friendly_name variable as the message title but I cannot figure out how to embed the variable in the last line of this snippet. Can anyone help?

esphome:
  name: "house-alarm"
  friendly_name: "House Alarm"
  on_boot:
    priority: 250
    then:
      - http_request.post:
          url: https://api.pushover.net/1/messages.json
          headers:
            Content-Type: application/json
          json:
            token: !secret pushover_api_token
            user: !secret pushover_user_key
            message: "Device just booted..."
            title: $friendly_name