You want to redirect your default
herokuapp.com domain to your custom domain.
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.