You can use any HTTP client to invoke your web API. In fact, you can invoke it directly from a web browser. In Visual Studio, start your application in debugging mode. Visual Studio will automatically open a web browser window with URL that points to http://localhost<portnumber>. The port number is automatically chosen by the development server. The public methods of the controller are called action methods. When the Web API framework receives a request, it routes the request to an action. To determine which action to invoke, the framework uses a routing table.
To invoke the custom GetCarsByMake method, browse to http://localhost:<portnumber>/api/cars?Make=Opel. Replace the <portnumber> with the actual port number. The exact result depends on the web browser you are using. The reason is that the different browsers send different Accept headers.