I guess for a systemwide variable you would need to edit the source code.
This was what I was aiming for. I ended up modifying about three files (or so) and modifying the ost_config database table but I managed to get what I was aiming for.not a system variable, but a ticket variable.
i.e: In the ticket details form, create a new label with the folliwing settings:
Label: Survey Link
Type: Choices
Variable: survey_link
Save, then change Survey label config, using the following:
then in your signature, just use the variable %{ticket.survey_link}
Good luck.
This would mean that a ticket would be created and then I would prompted but I also want the same to be made if I received a pipe'd in email ticket too. Hence why it should be System and not just ticket based.