Setting REST service URL parameters at runtime in Camel

Hi,
I want to pass some parameter values in my http GET request at runtime and
am not able to figure out a way to do this. The parameter is in my IN
message. I have pasted the code snippet below:-

public class DocumentCountRouteBuilder extends RouteBuilder {

public void configure() {

String docUID = "${in.body.documentID}";

from(documentServiceEndpoint)
.setHeader(Exchange.HTTP_METHOD, constant("GET"))
.setHeader(Exchange.HTTP_URI,
simple("http://rq3l2hps1:9081/V5/ECM/Rest/content/filenet/ClaimsTargetOS/TempClaimCenterDocument/Count?"
+"ᥠ_DocUID=${in.body.documentID}"))

.toF("http://url")
.transform(responseTransformer)
.marshal(format);

I can't seem to figure out how to set the value of GWCC_DocUID at runtime. I
don't understand how Camel variables get initialized since when I log the
value of ${in.body.documentID} it logs properly.
I am new to Camel. Please help.