Ksshaskpass is not meant to be executed directly, it will be called
by ssh-add it is not associated to a terminal.

To add it to the KDE autostart sequence you could for example create an
executable script with the following content in ~/.config/autostart-scripts/

/----------------
|#!/bin/sh
|# specify ksshaskpass as the ssh askpass program
|export SSH_ASKPASS=/usr/bin/ksshaskpass
|
|ssh-add
\----------------