Failed to load resource: the server responded with a status of 405 (Method Not Allowed)
Failed to load resource: No 'Access-Control-Allow-Origin' header is present on the requested resource.

public class PreflightRequestsHandler : DelegatingHandler
{
    protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        if (request.Headers.Contains("Origin") && request.Method.Method.Equals("OPTIONS"))
        {
            var response = new HttpResponseMessage { StatusCode = HttpStatusCode.OK };
            // Define and add values to variables: origins, headers, methods (can be global)
            response.Headers.Add("Access-Control-Allow-Origin", origins);
            response.Headers.Add("Access-Control-Allow-Headers", headers);
            response.Headers.Add("Access-Control-Allow-Methods", methods);
            var tsc = new TaskCompletionSource<HttpResponseMessage>();
            tsc.SetResult(response);
            return tsc.Task;
        }
        return base.SendAsync(request, cancellationToken);
    }

}

public static void Register(HttpConfiguration config)
{
    // Define and add values to variables: origins, headers, methods (can be global)
    // Enable global CORS
    config.EnableCors(new EnableCorsAttribute(origins, headers, methods));

    // Add handler to deal with preflight requests, this is the important part
    config.MessageHandlers.Add(new PreflightRequestsHandler()); // Defined above
    .
    .
    .
}

The browser can skip the preflight request if the following conditions are true:

The request method is GET, HEAD, or POST, **and**
The application does not set any request headers other than Accept, Accept-Language, Content-Language, Content-Type, or Last-Event-ID, **and**
The Content-Type header (if set) is one of the following:
 - application/x-www-form-urlencoded
 - multipart/form-data
 - text/plain

<system.webServer>
  <handlers>
    <remove name="ExtensionlessUrlHandler-Integrated-4.0" />
    <remove name="OPTIONSVerbHandler" />
    <remove name="TRACEVerbHandler" />
    <add name="ExtensionlessUrlHandler-Integrated-4.0" path="*." verb="*" type="System.Web.Handlers.TransferRequestHandler" preCondition="integratedMode,runtimeVersionv4.0" />
  </handlers>
</system.webServer>