Authenticate a USER in Asp.net MVC 5 without password

I want to authenticate a user with only user name and no password. My application does not have any user management data and I just want to create Identity with user details so that I can use it in the application.

I tried to copy the SingInAsync method to put this up

    private async Task InitializeUser()
    {
        var user = new ApplicationUser();
        user.Id = "abcd";
        user.UserName = "abcd";

        AuthenticationManager.SignOut(DefaultAuthenticationTypes.ExternalCookie);
        var identity = await UserManager.CreateIdentityAsync(user, DefaultAuthenticationTypes.ApplicationCookie);
        AuthenticationManager.SignIn(new AuthenticationProperties() { IsPersistent = false }, identity);
    }

But it tells me and error - that user ID cannot be found. Is there someway I can just authenticate the user by username and assign the Identity with some details??