code: public AuthRequest requestToken(String audience) {
          Asserts.assertNotNull(audience, "audience");
  
          String url = baseUrl
                  .newBuilder()
                  .addPathSegment(PATH_OAUTH)
                  .addPathSegment(PATH_TOKEN)
                  .build()
                  .toString();
          TokenRequest request = new TokenRequest(client, url);
          request.addParameter(KEY_CLIENT_ID, clientId);
          request.addParameter(KEY_CLIENT_SECRET, clientSecret);
          request.addParameter(KEY_GRANT_TYPE, "client_credentials");
          request.addParameter(KEY_AUDIENCE, audience);
          return request;
      }
docs: Creates a request to get a Token for the given audience using the 'Client Credentials' grant.
  Default used realm is defined in the "API Authorization Settings" in the account's advanced settings in the Auth0 Dashboard.
  <pre>
  {@code
  AuthAPI auth = new AuthAPI("me.auth0.com", "B3c6RYhk1v9SbIJcRIOwu62gIUGsnze", "2679NfkaBn62e6w5E8zNEzjr-yWfkaBne");
  try {
  TokenHolder result = auth.requestToken("https://myapi.me.auth0.com/users")
  .setRealm("my-realm")
  .execute();
  } catch (Auth0Exception e) {
  //Something happened
  }
  }
  </pre>
  
  @param audience the audience of the API to request access to.
  @return a Request to configure and execute.