There are certain steps that need to be followed for enabling CORS in ASP.NET Web API :

Step 1: We Have to install the CORS Package, and to install it , open Nuget Package Manager console and type and run command Install-Package Microsoft.AspNet.WebApi.Cors, this will install all the CORS packages.

Step 2: Now in Global.asax file, we define Default Route for the Web API

public class WebApiApplication : System.Web.HttpApplication
    {
        protected void Application_Start()
        {
            RouteTable.Routes.MapHttpRoute(
                      name: "DefaultApi",
                      routeTemplate: "api/{controller}/{id}",
                      defaults: new { id = RouteParameter.Optional }
                  );

            AreaRegistration.RegisterAllAreas();

            WebApiConfig.Register(GlobalConfiguration.Configuration);
            FilterConfig.RegisterGlobalFilters(GlobalFilters.Filters);
            RouteConfig.RegisterRoutes(RouteTable.Routes);
            BundleConfig.RegisterBundles(BundleTable.Bundles);
        }
    }

Step 3: Now to enable CORS , we have to add some line of code, just before the Default Route of API in Application_Start() function in Global.asax file

var cors = new System.Web.Http.Cors.EnableCorsAttribute("*", "*", "*");
GlobalConfiguration.Configuration.EnableCors(cors);

Here EnableCorsAttribute takes 3 parameters, which are

1. origins (Ex. "http://www.xyz.com")
2. headers: "*"
3. methods: "*"

Step 4:Now we will define the Controller and Action Method of the Web API, which goes like this

public class ValuesController : ApiController
    {
        public string Get(string fisrtName, string lastName)
        {
            return "FullName is : " + fisrtName + ' ' + lastName;
        }
    }

Note: Here the controller 'ValuesController', must inherits from ApiController, which is already present in MVC, and it provides basic functionalities for making Http request.