How to do load testing of Desktop application if some of its functionalities are on AWS server
How can I perform a load testing by any tool, if some of functionalities of any desktop application are getting computed on AWS server?
Load-testing is typically something you do with a server, not with a desktop application. You would write something that simulates the part of the desktop application that talks to the AWS server. If the AWS server is shared by multiple instances of the desktop application, you probably would need to simulate multiple instances of the desktop application.
There may be scale-related aspects of the desktop application that you want to test too, e.g. how big of a data set it can process. You may want to do that too, but you should not expect to find a "load-testing" tool that will help you with that.