NSURLRequest setting the HTTP header

for Swift

let url: NSURL = NSURL(string: APIBaseURL + "&login=1951&pass=1234")!
var params = ["login":"1951", "pass":"1234"]
request = NSMutableURLRequest(URL:url)
request.HTTPMethod = "POST"
var err: NSError?
request.HTTPBody = NSJSONSerialization.dataWithJSONObject(params, options: nil, error: &err)
request.addValue("application/json", forHTTPHeaderField: "Content-Type")
request.addValue("application/json", forHTTPHeaderField: "Accept")

You need to use a NSMutableURLRequest

NSMutableURLRequest* request = [[[NSMutableURLRequest alloc] initWithURL:url]
                                autorelease];

[request setValue:VALUE forHTTPHeaderField:@"Field You Want To Set"];

or to add a header:

[request addValue:VALUE forHTTPHeaderField:@"Field You Want To Set"];

I know its late but may help others , For SWIFT 3.0

let url = NSURL(string: "http://www.yourwebsite.com")
    let mutAbleRequest = NSMutableURLRequest(URL: url!)
    mutAbleRequest.setValue("YOUR_HEADER_VALUE", forHTTPHeaderField:"YOUR_HEADER_NAME")
    myWebView.loadRequest(mutAbleRequest)

 NSMutableURLRequest *request=[NSMutableURLRequest requestWithURL:url cachePolicy:NSURLRequestReloadIgnoringCacheData timeoutInterval:60];

[request setHTTPMethod:@"POST"];
[request setValue:postLength forHTTPHeaderField:@"Content-Length"];
[request setValue:@"application/x-www-form-urlencoded" forHTTPHeaderField:@"Content-Type"];
[request setValue:@"your value" forHTTPHeaderField:@"for key"];//change this according to your need.
[request setHTTPBody:postData];