PLASMA

$ echo "startplasma-x11" > ~/.Xclients
$ chmod a+x ~/.Xclients

from> https://askubuntu.com/questions/135483/how-to-configure-xrdp-to-start-cinnamon-as-default-desktop-session
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:

echo "startkde" > ~/.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