Issue
You want to redirect your default herokuapp.com domain to your custom domain.
Resolution
To redirect all requests to your custom domain you'll need to add a check at the application level that will detect if users are at the herokuapp.com domain and automatically redirect them to your custom domain.
The specific method on how to do this differs depending on the language being used, but there are many examples of how to do this on Stack Overflow that are easily searchable. Your specific languages documentation should also have details on how to do this.