title: Sending parameter request with RestSharp in C#. Getting Error: Required field \"expr\" missing in JSON body
body: <p>I am writing a simple calculation machine application in Windows <code>Form</code> application. I want to perform operations using math <code>js web api</code> (post), but where I call api I get this error:</p>
  
  <blockquote>
    <p>Error: Required field \"expr\" missing in JSON body" // in Response</p>
  </blockquote>
  
  <p>My code is here  : </p>
  
  <pre><code>   private void Equals_Click(object sender, EventArgs e)
  
      {
          var client = new RestClient("http://api.mathjs.org/v4/");
          var request = new RestRequest("/expr", Method.POST);
          var deger = txtCevap.Text; // txtCevap.Text is my calculator parameter
          var json = JsonConvert.SerializeObject(deger);
          request.AddParameter("application/json; charset=utf-8", json, ParameterType.RequestBody);
          request.RequestFormat = DataFormat.Json;
          IRestResponse response = client.Execute(request);
  
      }
  </code></pre>
  
  <p>Can you help me in this topic? Thanks.</p>