That entirely depends on what you mean by "test this."
Typically, you use Fiddler to capture (and potentially modify) traffic between a client and the server. In other cases, you might use the Composer
tab to generate a new HTTP request directly and send it to the server, using Fiddler's Inspectors to analyze the server's response.
Perhaps you're asking: "What content do I need to add to the Composer tab to issue a request that matches the target API"-- Unfortunately, that's really a question about WCF and not about Fiddler. The simplest approach would be to simply generate a client which talks to this service and use Fiddler to capture its traffic. You can then drag/drop such a request from Fiddler's Web Sessions list to the Composer to clone it and optionally modify it before resending.
Check out the Telerik Platform - the only platform that combines a rich set of UI tools with powerful cloud services to develop web, hybrid and native mobile apps.