Use RedirectToAction in Web API

I am using RedirectToAction in my ASP.Net WebAPI application and I tried the following one.

return RedirectToAction("AuthenticateUser", "AuthenticationServiceWebApi", new RouteValueDictionary
                                                                    {
                                                                        {"userName", model.UserName},
                                                                        {"password", model.Password}
                                                                    });

This generates the redirection as below.

127.0.0.1:81/authenticationservicewebapi/authenticateuser/admin/admin@123

But, since I am using WebAPI, I need to be the URL like below.

127.0.0.1:81/api/authenticationservicewebapi/authenticateuser/admin/admin@123

How do I do this?