apm sun DC authentication fail

Hi,

 

I have a parent and child domain controller let say f5dc.com , sub.f5dc.com . I configured an AD authentication on the apm and I should login a web portal with users that created each parent and child domain. I can login with user that was created on the parent domain but I could not login with user that was created on the sub domain. I enabled the split domain on the login section but I still could not login to the portal with a subdomain user.When I try to login I type username as " sub\testuser ". I am not sure it is correct.?

 

Thank you.