Hi UserAP,
I am not a Target expert, so not sure if the answer below can help.
But have you tried using Processing Rules to pass the Activity ID from Target to an eVar? I just did a quick spin off on Target in my sandbox, and I can see that the Target Session ID is enabled. I am sure there will be a Context Data available for Target Activity ID which you can copy to an eVar.
Regards,
Ritesh Gupta
www.thelearningproject.in