https://learn.microsoft.com/en-us/dotnet/api/system.web.security.activedirectorymembershipuser.provideruserkey?view=netframework-4.8.1
ActiveDirectoryMembershipUser.ProviderUserKey Property (System.Web.Security) | Microsoft Learn
Gets the user identifier from the Active Directory data store for the membership user.
property systemweb securitymicrosoftlearn