The singleton class instance is active in a single session. When you display a pop-up and then call the target application, you are jumping from session to another. Any way, one way to accomplish what you are trying to do, you can pass the account group as a URL parameter (good old ASP & BSP technique).
I'm wondering why do you have this pop-up window in the first place? Why not just make the BP account group mandatory so that users will have to pick a value always? If you don't want all the values in the drop-down, you can filter them out in the feeder class GET_DATA method.