https://learn.microsoft.com/zh-tw/dotnet/api/system.web.security.formsauthentication.getredirecturl?view=netframework-4.8
FormsAuthentication.GetRedirectUrl(String, Boolean) Method (System.Web.Security) | Microsoft Learn
Returns the redirect URL for the original request that caused the redirect to the login page.
system webstringbooleanmethodsecurity