Heroku support custom domain for free Dynos, but withou the SSL certificate, is not so you'll end up serving your app on your custom domain via HTTP protocol, not so great.
You can work around by letting Cloudflare handle the SSL part.
First, in your Heroku application setting, add a new custom domain. Heroku will provide a DNS record so you can set it up on Cloudflare.
In Cloudflare DNS, add a new CNAME record with the information Heroku just provided. Don't forget to turn on Full SSL and Always HTTPS mode for your domain.
Now you'll get your application serving with HTTPS on a custom domain.
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