HomeController.cs

[AllowAnonymous]
public async Task<IActionResult> Login(string username, string password)
{
    if (IsValidUser(username, password))
    {
        var claims = new List<Claim>(2);
        claims.Add(new Claim(ClaimTypes.Name, username));

        claims.Add(new Claim(ClaimTypes.Role, "GroupThatUserIsIn", 
            ClaimValueTypes.String, "IHaveIssuedThis"));

        await HttpContext.SignInAsync(
            CookieAuthenticationDefaults.AuthenticationScheme,
            new ClaimsPrincipal(new ClaimsIdentity(claims, 
            CookieAuthenticationDefaults.AuthenticationScheme)));

        return RedirectToAction("Index");
    }
    return View();
}

private bool IsValidUser(string username, string password)
{
    return username == "foo" && password == "bar";
}