Hi Steven,
Even though system gives you return a bad request. I does not mean that is CORS issue. Please login into backend system goto tcode : /IWFND/ERROR_LOG. Check if system is logging error here. If it is logging , select the error and click on active source . You can see the source code where there is exeception. You can even paste the source here with class name and method , so we can help you why you are getting that error.
Thanking you
Vengaiah