I have bad news unfortunately: You need to create the apps from the Heroku side. Unfortunately because they manage the billing on a per-app basis, and control that, it's out of our hands (it is frustrating for us also, and we do migrate folks off of Heroku or other partners like Azure due to limitations imposed upon us).
So, I'd recommend not removing the application from Heroku, as that will cease billing and you'll not have a paid account at all. To create new applications, you'll also need to create them individually at the Heroku side as well. Again, more a limitation when we integrated with them.
We are looking at bringing other Raygun products (Real User Monitoring and APM) to the Heroku Addon store so we can have the team investigate if there may have been improvements Heroku have added in recent years to make this nicer, but for the time being you'll need to provision the apps in Heroku to manage it all.
Sorry it's not better news. Happy to discuss in more detail and discuss billing options (if there's any chance of moving from Heroku doing it) here: firstname.lastname@example.org.