[Question] Connect to web application

Hi everyone,

At present, I'm making a website by using C# MVC5 with these functions:

- Login 

- Managing users

public ActionResult Login(LoginInfo loginInfo);

In user management function, I have these function:

public JsonResult GetUserInfo(string userId);
public JsonResult CreateUser(User user);

My website has been completed, and I want to make a desktop application which helps admin to connect to my website to use those functions above.
I still don't know how to do because to execute those functions above, user must be logged in. 
Therefore, my question is : How can I make my desktop application to connect to my website, use login function to login and manage user ?

Can anyone help me please ? 

Thanks in advanced.