Running a MonoBehavior script A with a coroutine in another Script B

Hi, I am a newbie in OOP and Unity/C#. I am using WWWForm in a script called PostData.cs that posts data on a server. Here is the code for PostData.cs

 using UnityEngine;
 using System.Collections;
 
 public class PostData : MonoBehaviour {
     
     public string SecretKey = "this works!";
     
     void Start ()
     {
         PostData();
         }
     
     public void PostData()
     {
         string url = "http://place/somefile/awesome.php"; 
       
         WWWForm form = new WWWForm();
 
         //form.AddField(,);
         form.AddField("name", abc);
         form.AddField("school", "cdf");
                 form.AddField("rating", 5);
         form.AddField("yes/no", 1);
         form.AddField("description", "goals");
         
         WWW www = new WWW(url, form);
 
         StartCoroutine(WaitForRequest(www));
         
     }
     public string SecretKey ()
     {
         return secretCodeWord;
     }
 
         public IEnumerator WaitForRequest(WWW www)
     {
         yield return www;
         if (www.error == null)
         {
             Debug.Log("WWW Ok!: " + www.text);
         } else {
             Debug.Log("WWW Error: "+ www.error);
         }    
     }    
 }

Now I want to run this script coroutine that writes the data to the server in another script called TestPost.cs. I have tried to write it as follows

 using UnityEngine;
 using System.Collections;
 
 public class TestPost : MonoBehaviour {
     
     // create an instance
     PostData postIt = new PostData();
     
     void Start () {
             Debug.Log ("started");
             // calling the simple function to test
             Debug.Log(postIt.getCodeWord());
             // calling the required function
             postIt.PostData();
             Debug.Log("something happened");
     }
 
 }

When I run the TestPost.cs file I get the following output:

 started
 
 this works!
 
 NullReferenceException
 UnityEngine.MonoBehaviour.StartCoroutine (IEnumerator routine)

So calling the simple string return function works, but the coroutine is not called. Does anyone have an idea of what I am doing wrong? Any kind of help will be appreciated. thanks

PS: Sorry, I ended up asking a very lengthy question!!