Found this on the net, on this url: http://sigkillit.com/tag/xrdp
Customize Desktop Environment for xRDP Session
If you do not want to use the default desktop environment, you can customize it by creating a .Xclients file (X is capital!!!) in your home directory to launch the desktop environment you want and making it executable. In order to do this, open a terminal and run one of the following commands
Gnome 3:
echo "gnome-session" > ~/.Xclients
chmod +x ~/.Xclients
sudo systemctl restart xrdp.service
Gnome Fallback:
echo "gnome-fallback" > ~/.Xclients
chmod +x ~/.Xclients
sudo systemctl restart xrdp.service
KDE 4:
echo "startkde" > ~/.Xclients
chmod +x ~/.Xclients
sudo systemctl restart xrdp.service
KDE 5:
echo "startplasma-x11" > ~/.Xclients
chmod +x ~/.Xclients
sudo systemctl restart xrdp.service
MATE:
echo "mate-session" > ~/.Xclients
chmod +x ~/.Xclients
sudo systemctl restart xrdp.service
Cinnamon:
echo "cinnamon" > ~/.Xclients
chmod +x ~/.Xclients
sudo systemctl restart xrdp.service
Xfce4:
echo "startxfce4" > ~/.Xclients
chmod +x ~/.Xclients
sudo systemctl restart xrdp.service