如何在成为旧版ASP.NET MVC应用程序的一部分时在Web API 2中对用户进行身份验证？

I have a Web API which is currently used by AngularJS apps in an ASP.NET MVC web application. The MVC application is utilizing ASP.NET Forms Authentication as authentication mechanism. How should I authenticate a user of the Web API when the client is not the web client but e.g. a stand-alone service. What I've done right now is adding a login method to the Web API which gives anyone with right credentials access:I have a Web API which is currently used by Ang