You want to send read-only Postgres queries to a follower database instead of sending them to the leader database.
The distinction of which queries go to which database will need to be handled within your application code. Though providing application-specific code consultations is outside the scope of the Heroku Support Policy, you'll likely need to make use of the
HEROKU_POSTGRESQL_<COLOR>_URL configuration variables to avoid hard-coding the various database credentials within your app's code.
It's important to note that follower databases can fall slightly behind the leader databases. Depending on your use case, this may or may not be an issue.