Your problem isn’t with jquery, it’s in how CORS works. Your beforeSend callback was probably working as expected… but browsers won’t send custom headers in preflight requests, no matter what. This is by design; the purpose of the preflight request is to determine what information the useragent (browser) is permitted to send beyond the “simple” stuff defined in the CORS spec. Thus, for the useragent to send any non-simple data (such as your custom header) as part of the preflight request is self-defeating.
To instruct the useragent to include your custom header in the actual CORS request, include a Access-Control-Allow-Headers header in your preflight response. It’s worth noting that if you’re not overly concerned with what headers the useragent transmits, I believe you can just echo back the value of the Access-Control-Request-Headers request header field as the value of the Access-Control-Allow-Headers you send in the response.
You may also want to include some of the other Access-Control-Allow-* headers defined in the syntax section of the spec.
See also CORS – How do ‘preflight’ an httprequest?
See also Mozilla’s CORS preflight example, which shows these headers in action.