title: Use REST API service and connect with token endpoint provided by keycloak
body: <p>I want to link a rest service with an authentication provided by keycloak.
  Keycloak requires a some variables in body</p>
  <ul>
  <li>username</li>
  <li>password</li>
  <li>grant_type</li>
  <li>client_id</li>
  <li>client_secret</li>
  </ul>
  <p>I couldn't find the part in synapse where I can provide these informations</p>
  <p><a href="https://i.stack.imgur.com/rS7ug.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/rS7ug.png" alt="Add rest service in synapse" /></a></p>