that was how to achive this in restlet 2.0.x I'm assuming that you are using a more recent version? at 2.1.x try
Series<Header> series = (Series<Header>)getRequestAttributes().get("org.restlet.http.headers");
series.getFirst("LtpaToken2");
there was mention of a short cut method, so that you did not need the magic String org.restlet.http.headers
but I'm not sure which version that was / is being introduced in.