So here's how I got it to work (I changed it to use "GET" instead of "POST)". The deepl doc should get updated. Not sure why POST did not work.
~$ curl -i -X GET \
-H "Authorization:DeepL-Auth-Key mysecretkey:fx" \
'https://api-free.deepl.com/v2/translate?text=hello%20world&target_lang=EL'