All Heroku Postgres databases have a corresponding Heroku application. Whether or not you use the app, your database is attached to it and it holds an environment variable containing the database URL.
Log in the Heroku Postgres interface. You should see a list of your databases.
Switch to the Apps tab and click on the app that corresponds to your database. It will likely have the same name as your database.
Click on the Settings tab, then click Reveal config vars.
Copy the value for DATABASE_URL.
It should look something like this:
From your dashboard, navigate to your Project Settings by clicking the gear icon.
Copy the URL from Heroku, paste it into the input box in Chartio, and click Connect.
Heroku wisely enables PostgreSQL SSL connections by default. Every data connection Chartio makes is encrypted this way.