You can use Microsoft.AspNet.WebApi.Cors, a Nuget package from Microsoft. Follow these step:

1. install package for your API project.

2. add one line code to your App_Start/WebApiConfig.cs file to enable the function: config.EnableCors();

   using System.Web.Http;
   namespace WebService
   {
       public static class WebApiConfig
       {
           public static void Register(HttpConfiguration config)
           {
               // New code
               config.EnableCors();
               config.Routes.MapHttpRoute(
                   name: "DefaultApi",
                   routeTemplate: "api/{controller}/{id}",
                   defaults: new { id = RouteParameter.Optional }
               );
           }
       }
   }
   

3. use attribute in your controller or action like this:

   using System.Net.Http;
   using System.Web.Http;
   using System.Web.Http.Cors;
   namespace WebService.Controllers
   {
       [EnableCors(origins: "http://mywebclient.azurewebsites.net", headers: "*", methods: "*")]
       public class TestController : ApiController
       {
           // Controller methods not shown...
       }
   }
   

You can get more info here

1=> var cors = new EnableCorsAttribute("http://mydomain", "*", "*"); config.EnableCors(cors); I wrote this code in web config file as global level but its working fine when I am calling my web api from client side its allowing only for specified domain

2=> This is my Client side code

<script src="https://cdnjs.cloudflare.com/ajax/libs/knockout/2.2.0/knockout-min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
$ $.ajax({
                url: "http://localhost:49676/Account/ValidLogin",
                method: "get",
                data: Employee,
                dataType: "json",
                //contentType: "application/json",
                success: function (data) {
                    GetAllEmployee(data);

                    alert("success");
                    console.log(data);
                },
                error: function (err) {
                    alert("error");
                }

Here is my server side code

protected void btnDelete_Click(object sender, EventArgs e) {

        string strUrl = string.Format("http://mydomain/account/validlogin?username=admin&password=admin");
        HttpWebRequest requestObjectGet = (HttpWebRequest)HttpWebRequest.Create(strUrl);
        requestObjectGet.Method = "GET";
        HttpWebResponse responseObjectGet = null;
        responseObjectGet = (HttpWebResponse)requestObjectGet.GetResponse();

        string strresulttest = null;

        using (Stream stream = responseObjectGet.GetResponseStream())
        {
            StreamReader sr = new StreamReader(stream);
            strresulttest = sr.ReadToEnd();
            JavaScriptSerializer js = new JavaScriptSerializer();
            string StrToken = js.Deserialize<string>(strresulttest);
            Session["ApiToken"] = StrToken;

}