url_for with _external=True on heroku doesn't append the server name on the URL
To generate the correct URL, these two Flask configuration values need to be set:
SERVER_NAMEto the name of your website,- and
PREFERRED_URL_SCHEMEtohttps.
See the documentation for url_for() and how to configure a Flask application.
EDIT: Additionally, the parameter in url_for should be _external and not external.