title: Retrofit: How to send a POST request with constant fields?
body: <p>I want to sent a simple POST request with one actual parameter:</p>
  
  <pre><code>@POST("/token")
  @FormUrlEncoded
  void extendSession(@Field("refresh_token")final String refreshToken);
  </code></pre>
  
  <p>But this request should also send some constant values requested by the server such as <code>client_id</code>, <code>client_secret</code> and <code>grant_type</code> which are constant and should not be part of the application API.</p>
  
  <p>What is the best way to do this?</p>