How to kick off a login script in Citrix

I have been tasked with automating a script for when a user logs in to a Citrix Session. The script basically modifies an .ini file in the user’s Windows profile that is created the first time they log in to Citrix. Trouble is, I do not know of a way to have Citrix kick off a login script when the user logs in. Maybe I’m just missing something. Any ideas?