I am on Arch Linux (5.15.13-arch1-1) and Xfce (xfce4-session 4.16.0 (Xfce 4.16)).
I use the following command to run Picom, and a similar command works just fine in my autostart script for XMonad.
picom -b --config ~/.config/xfce/picom/picom.conf --experimental-backends
However, creating an entry in session startup doesn’t seem to do anything.