I was having trouble with my domain being redirected to my app. At the time I had my domain from GoDaddy although I think it applies to any DNS.
Here are the steps to set it up.
# Run this command in your terminal heroku domains:add www.example.com
Then go to your DNS records on GoDaddy or your Domain Name Registrar.
Create a CNAME record: Host: www Points To: example.herokuapp.com
Also add a forwarding record to redirect example.com to www.example.com
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