You cannot automate this application effectively without installing Pega Robotics on the Citrix machine(s). You might be able to complete a really simple RPA project using screen OCR, but that would be really limited in what you can do.
To automate a Citrix application, you must have a means of interrogating the application. This usually means having one of the Citrix servers getting an installation of Pega Robotics Studio.
For your end-users (or bots if using unattended RPA), you would need Runtime installed on both the desktop and the Citrix server (assuming you needed to automate applications within Citrix as well as those outside of it). Runtime on the desktop would launch Runtime on the server and communicate between them via the Citrix Channel.