Disabling default settings welcome window

[Prunk Dump]

Hello XFCE team !

I now use XFCE in entreprise with Debian 8 Jessie (xfce-4.10.1).

When my users log in for the first time a windows appear asking for
choosing between something like "default panel" and "empty panel".

When my users click on the "empty" choice they often have trouble
using XFCE and I need to reset their account. So I want to force my
users using the default settings and disable this popup window.

I cannot find any config file to set this.

Where are the default setting stored ? I have nothing in /etc/skell.
Maybe in /etc/xdg/xfce4 but where can I find documentation of there
config files.

Can someone help me ?

Baptiste.