Modeling a shaker mounted to the roof

[lotsofquestions]

I'm trying to model a structure in SAP2000 that has a shaker mounted to the underside of the roof slab. I've defined a time history acceleration function to represent the shaker input but I can't figure out how to apply it to a specific node on the roof of my structure. thread801-116070 says that SAP automatically applies a time history acceleration at the base of my structure. How do I apply it at specific nodes instead of at the base of my structure?