To sign in with Heroku to Postgres Monitor, you can use the Heroku CLI or the Heroku dashboard.
Run this command to sign in with Heroku:
$ heroku addons:open postgres-monitor
Open the Heroku dashboard and navigate to the Heroku app that Postgres Monitor is attached to. Go to the Resources tab and click on the Postgres Monitor add-on in the bottom section. This will open the Postgres Monitor dashboard and sign you in automatically.