unable to send post request using urlrequest

I'm just trying to send simple variable to server using post request and I want to do it manually. Here is my code

   guard let url = URL(string: urlString+"/someapi") else {
        return
    }
    var request = URLRequest(url: url)
    request.httpMethod = "POST"
    request.setValue(String(3), forHTTPHeaderField:"Content-Length")
    request.setValue("application/x-www-form-urlencoded", forHTTPHeaderField: "Content-Type")
    request.setValue("text/plain", forHTTPHeaderField: "Accept")
    var httpBody:Data=Data()
    httpBody.append(contentsOf: "f=1".data(using: .utf8)!) //f is key & 1 value
    request.httpBody = httpBody
    let onComplete = {
        (data:Data?,urlresponse:URLResponse?,error:Error?)->Void in
        //do nothing
        if let _ = error {
            print("Error occured")
        }
    }
    let task = session.dataTask(with: url, completionHandler: onComplete)
    task.resume()

I'm using flask server to test this out. On flask side I analysed the request here is the it's header content

Accept: */*
Accept-Language: en-US,en;q=0.9
Connection: keep-alive
Accept-Encoding: gzip, deflate
User-Agent: Find%20Doctor/1 CFNetwork/1333.0.4 Darwin/21.4.0

And also flask server is receiving GET request instead of POST and also variables are not showing in server.