How can I use this in Elastic Cloud?
It's no different, just provide a username and password for the cluster and then change the URL.
See https://www.elastic.co/guide/en/cloud/current/ec-security.html for more details on authentication.
And whats the URL then? And where/how can I provide the username+password? How does this call know where my data is located...
https://www.elastic.co/guide/en/cloud/current/ec-getting-started-connect.html
If you are not familiar with curl, then -u username:password is the flag. You may also want to consult the manual for it.
That is what the --data-binary @accounts.json means, it's referring to a file called accounts.json.
Yes, but how can the Elastic Cloud API Console know where I have the file locally on my machine?
It doesn't, you need to use curl to send it.