The custom Content-Type header you’re sending causes your request to be preflighted, which means an OPTIONS request, containing some metadata about the POST request that is about to be dispatched, will be sent before the actual POST request.
Your server needs to be prepared to deal with this OPTIONS request. You haven’t specified what the server is written in, but with express for example, you can register a middleware that intercepts all ‘OPTIONS’ requests, sets the Access-Control-Allow-Origin: * and Access-Control-Allow-Headers: Content-Type headers, and responds with 200.
If it is possible for you to make the request using a ‘Content-Type’: ‘text/plain’ header, that would solve your problem. Alternatively you could use something that bypasses XHR entirely, like JSONP.