VPN and SSO authentication

Hello

I'm connecting a remote branch office to the main office via VPN (PPTP) connection. In the main office there is a XG Firewall, in the branch office a mikrotik router. I need to authenticate my remote users by using a Domain Controller located in the main office. Currently, in the main office the Active directory and SSO integration works correctly. I can authenticate the users in the remote office with the AD, but in the XG Firewall, the traffic of the remote users appears with the VPN user, not the AD user. How can I mark the traffic of the remote users with the AD user, not with the VPN user?

Thanks