SMS problem - CORS?

I am using a service for SMS (
Basically this is the relevant code from appStudio (I put the commented out ones in the Project ExtraHeaders property) :"POST", "");
//xhr.setRequestHeader("Content-Type", "application/json");
//xhr.setRequestHeader("Authorization", "Basic " + btoa(""));
//xhr.setRequestHeader("Cache-Control", "no-cache");


This is what the Extra Headers property for the project looks like:

I am getting this error - any ideas?

A sample project with a demo acct: (7.7 KB)