In the web.config there is MSReportingServer setting that points to the report server.
In order to set it in the code dynamically you may override it in your 'ReportService.svc.cs' file like in the following code:
public class ReportService : ReportServiceRemote
protected override string GetServiceUrl()
protected override System.Net.ICredentials GetServiceCredentials()
, where "MYURL" is the URL you want to set.
parameters credential credentials