Issue with WWW/UnityWebRequest when sending JSON to server

Hi all, I have a problem when using either WWW/UnityWebRequest to send JSON data to my server.

I have tested my server request API on Postman, and it works. The server sends back text msg (like "Success"). But it doesn't work on Unity3D. I have tried both WWW and UnityWebRequest to send my JSON data. The code shows below, (I only posted UnityWebRequest one),

btw: the JSON I want to send is {"data", AnotherJSON}, thus I AddField by AddField("data", jsonstr)

  IEnumerator PostData(){
             User usr = new User();
             usr.name = "abc";
             usr.email = "abc@gmail.com";
             usr.age_range = 23;
             usr.gender = "male";
             usr.location = "U.S.";
             string jsonStr = JsonConvert.SerializeObject(usr);
     
             WWWForm form = new WWWForm();
             form.AddField("data", jsonStr);
             //Dictionary<string, string> headers = form.headers;
             //headers["Content-Type"] = "application/json";
             UnityWebRequest uwr = UnityWebRequest.Post(addUerURL, form);
             uwr.SetRequestHeader("Content-Type", "application/json");
             yield return uwr.Send();
     
             string s = uwr.downloadHandler.text;
             if(uwr.isError){
                 Debug.Log(uwr.error);
             }else{
                 Debug.Log("Success? " + s);
             }
 }

The User class is just a public class contains basic info using default constructor. The output goes into "No Error" and prints "Sucess? 400 Bad Request" which indicates my request are malformed.

And when I use "WWW www" as sending my request, I got the same, 400 Bad Request.

Anyone can help me out???? A lot thanks in advance.