SSL-VPN Web portal - Single Sign On RDP with domain

Hello,

I have set up SSL-VPN web portal for our employees, the User group for VPN users is loaded remotely from Active Directory group, so I don't have to create users manually.

I have created a bookmark for our terminal server via RDP and checked Single Sign On - SSL-VPN Login. 

Users can login to Web portal, using just their username, without domain specified, which is fine, but the Single sign on doesn't work because the RDP needs domain specified to login domain user and not local user. 

Is there any way around this?

Thank you.