POST request to RESTful APIs

I am trying to make a POST request in Unity script. The header should contain the key 'Content-Type' that is set to 'application/json'. The input key is "email".

So here's my script:

 private static readonly string POSTWishlistGetURL = "http://mongodb-serverURL.com/api/WishlistGet";

 public WWW GET()
 {
     WWWForm form = new WWWForm();
     form.AddField("email",  "abcdd@gmail.com");

     Dictionary<string, string> postHeader = form.headers;
     if (postHeader.ContainsKey("Content-Type"))
         postHeader["Content-Type"] = "application/json";
     else
         postHeader.Add("Content-Type", "application/json");

     WWW www = new WWW(POSTWishlistGetURL, form.data, postHeader);

     StartCoroutine(WaitForRequest(www));
     return www;
 }

 IEnumerator WaitForRequest(WWW data)
 {
     yield return data; // Wait until the download is done

     if (data.error != null)
     {
         MainUI.ShowDebug("There was an error sending request: " + data.error);
     }
     else
     {
         MainUI.ShowDebug("WWW Request: " + data.text);
     }
 }

I keep getting data.error = 400: Bad Request. How do you properly create a POST request?