I've just turned on the CDN support at the bottom then it showed me "my_own_bucket.objects.cdn.dream.io" which seems to be the right url to use, and I went back to my DNS provider and I re-enter "my_own_bucket.objects.cdn.dream.io" just to make sure everything is ok.
That's all what I did so far. now if I go to my bucket settings -> Aliases I can put a subdomain but the dropdown menu still empty ! so I am not able to choose a domain!
In the documentation there is a note saying : "If you’re using a third-party DNS, be sure to manually copy the CNAME from the panel to the name servers since it won’t automatically propagate." I am not sure what does this mean exactly! maybe I should do something to make the dropdown recognize my domain name ?