What kind of a dynamic load are you looking to apply? A vibrating load due to machinery on one of the floors? Or a ground excitation due to an earthquake?
Either way, the type of building isn't really going to affect the application of the load. For applying a seismic load, see example 29 of the examples manual. For applying a load induced by vibrating machinery, see example 22 of the same manual. If it is a randomly varying force, see example 16.
Unfortunately, all of the above examples show the specification of the data thru the text file only. There are no examples that show how to assign this data using the STAAD GUI. If you face any difficulty assigning them thru the GUI, let us know.
Also, if it is a 80-storey building, the size of the model could be so large that, coupled with the significant computing resources needed for a time history analysis, you may face difficulties in getting the analysis to be completed. If so, you may have to consider reducing the model size.