auth_service_sign_in_screen_redirection.cs

public class HomeController : Controller
{
  [HttpGet]
  public ActionResult Index(string platform, string provider, string token)
  {
    string url;

    if (provider == "apple")
      url = "https://appleid.apple.com/auth/authorize" +
        "?client_id=com.your.app" +
        "&redirect_uri=https://auth.yourapp.com/callback" +
        "&response_type=code%20id_token" +
        "&response_mode=form_post" +
        "&scope=email" +
        "&state=" + (platform ?? "unknown") + ';' + token;

    else if (provider == "google")
      url = "https://accounts.google.com/o/oauth2/auth" +
        "?client_id=XXXXXXXXXX.apps.googleusercontent.com" +
        "&redirect_uri=https://auth.yourapp.com/callback" +
        "&response_type=id_token" +
        "&response_mode=form_post" +
        "&scope=email" +
        "&state=" + (platform ?? "unknown" ) + ';' + token;

    else return this.BadRequest();

    return this.Redirect(url);
  }
}