POST request using WWW class with header

I am trying to make a POST request to restful web APIs in Unity.

The header would be Content-Type: application/json

An example of the raw data input is, where data is the key and json string is the value:

{"data":{"username":"name","email":"email@gmail.com","age_range":21,"gender":"male","location":"california"}}

Here's my script:

 private static readonly string POSTAddUserURL = "http://db.url.com/api/addUser";

 public WWW POST()
 {
     WWW www;

     Hashtable postHeader = new Hashtable();
     postHeader.Add("Content-Type", "application/json");

     WWWForm form = new WWWForm();
     form.AddField("data", jsonStr);
     www = new WWW(POSTAddUserURL, form);

     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);
     }
 }

How do I send the request using WWW class with both form and header? Or, just in general, how to i send this kind of post request?