*** Note ssh-add will NOT use GUI tools like lxqt-openssh-askpass 
*** when it is attached to a terminal. 

* To use lxqt-openssh-askpass start ssh-agent with -a <SOCKET_PATH> e.g in .xinitrc
ssh-agent -a "/tmp/${USER}-ssh-agent"

* Then set two variables e.g in ~/.config/lxqt/session.conf:
SSH_ASKPASS=lxqt-openssh-askpass
SSH_AUTH_SOCK=<SOCKET_PATH>
then run ssh-add from lxqt-runner or a .desktop entry