Post Request in wkwebview using form submit does not work in some cases as no referrer headers sent out

Question

I am performing Post Request in wkwebview by submitting a form works good when I try to login some social websites as described this question's answer, except Facebook mobile site. Facebook gives me the error message do no enter your password on sites not located at facebook.com which maeans facebook is able to detect referrer, may be through the headers sent out. m.facebook.com needs header value of referrer to be m.facebook.com only and I am not abe to set the headers in my post call I tried to modify my request in decidePolicyForNavigationAction

    func webView(webView: WKWebView, decidePolicyForNavigationAction navigationAction: WKNavigationAction, decisionHandler: (WKNavigationActionPolicy) -> Void) {

    let urlString = navigationAction.request.URL!
    if(urlString.absoluteString.rangeOfString("https://m.facebook.com/login/?email") != nil){    // which decides it is a post req and is for facebook

        let req =  navigationAction.request;

        webView.navigationDelegate = self
        webView.loadRequest(req);
              decisionHandler(WKNavigationActionPolicy.Cancel)        
    }else{
             decisionHandler(WKNavigationActionPolicy.Allow)
    }
}

then I got to know that this request is not mutable ,so cannot be modified , so I tried it with the mutable copy of it

        let req = NSMutableURLRequest(URL: navigationAction.request.URL!) 
      and this                 
        let req =  navigationAction.request.mutableCopy();
      instead of 
        let req =  navigationAction.request;

but none of them worked. I asked the same in the comments of the above question (whose link I provided), but no response, is there any work around for this?

If anyone could help me in this, I'll be more then Happy Thanks in advance :)