Heroku validation applications, set Meteor ROOT_URL correctly when deploying to view the application

Question

Example app.json in the root of the directory. I am trying to figure out how I can take HEROKU_APP_NAME and set the ROOT_URL variable, so I don’t have to manually set it every pull request.

{
  "name":"Advanced App",
  "scripts":{
    "postdeploy":"rake db:setup && bin/bootstrap"
  },
  "env":{
    "HEROKU_APP_NAME": {
      "required": true
    },
    "HEROKU_PARENT_APP_NAME": {
      "required": true
    }
  }
}