You should not place the servlet within Worklight Server.
For your servlet to work while in Worklight Server, you will need to manually edit the web.xml file, otherwise the server will not recognize the servlet and will not listen to any incoming requests.
However, the web.xml is part of the .war file; this file is re-generated upon each build operation in Worklight Studio (in Eclipse, where the Worklight Server instance is part of), thus the web.xml is re-generated as well and your additions will be lost.
You need to deploy this servlet to some web server.
You can then reach it by using an HTTP adapter just like any other HTTP backend system. See the documentation for HTTP adapters.