How to set session im web api call to my MVC5 Pro Login Page

Question

public ActionResult Login(MST_Users login)
{
string UserName = login.UserName;
string Password = login.Password;

var client = new HttpClient();
var response = client.PostAsJsonAsync("http://localhost:1724/api/LoginAPI", login).Result;

Session["USERID"]=?;

 

}