WWWForm doesn't work on iOS

The following code works on Windows Desktop and Android platforms, but does not work on iOS:

 IEnumerator Send(string data)
     {
             WWWForm form = new WWWForm();
             form.AddField("data", data);
 
             WWW www = new WWW(baseurl + "/api-data.php", form);
 
             yield return www;
     }

On iOS the function seems to exit immediately without sending/receiving any data.

How can I fix this for iOS?