[AllowAnonymous]
[HttpPost("Login")]
[Produces("application/json")]
public IActionResult Index(string returnUrl)
{
    var redirectUrl = Url.Action("Callback", "ExternalLogin", new { ReturnUrl = returnUrl });
    var properties = _signInManager.ConfigureExternalAuthenticationProperties(Saml2Defaults.Scheme, redirectUrl);

    return Challenge(properties, Saml2Defaults.Scheme);
}