Securing new WCF service which is part of MVC application using forms authentication

Hi,
I do have exisiting MVC 5 (.NET 4.5.1) ASP.NET web application which has in web.config configuration for FormsAuthentication. I have this part solved in code and it works.
My question is, how to add, additional Visual studio project inside existing solution, which will be WCF / SOAP service based on HTTP and/or HTTPS requests (XML) for querying additional data from my web app, using my kind of API. I want this WCF service to use same forms authentication from first project.
Please help me write simple web.config for WCF service with this formsauth from existing project. And some C# code as example of checking auth in SOAP methods (if neceserry).
Thank you very much!