curl is a command line thing. Like Postman for people that do coding stuff 
You can’t put that into the body of the API (although Blockspring did provide a way to “curlAnything” but I could never get it to work and I ain’t paying for it now).
It is very frustrating when API providers just throw a bunch of language implementations at you.
However … cho had sorted the API for plivio here …
You are going to have to think about how that callback url works.