When you say "EMPro of ADS", do you mean the EMPro user interface, or the FEM solver driven from the ADS user interface?
For the EM simulation based on ADS layout, you need to define the parameters in your *layout*. Then, using that parameter in EM simulation is easy: create an emModel for your layout, and set/sweep the parameters of that emModel using a schematic.
But how to define and use parameters in layout? Of course, your layout cell needs parameters: File > Design Parameters. Changing your layout based on these parameters depends on the elements you use in layout: components layouts (which are already parametric, and you just need to use the cell parameters in the layout element parameters) or simple shapes (which can be parameterized from EM > Component > Parameters).
For the EM simulation based on ADS layout, you need to define the parameters in your *layout*. Then, using that parameter in EM simulation is easy: create an emModel for your layout, and set/sweep the parameters of that emModel using a schematic.
But how to define and use parameters in layout? Of course, your layout cell needs parameters: File > Design Parameters. Changing your layout based on these parameters depends on the elements you use in layout: components layouts (which are already parametric, and you just need to use the cell parameters in the layout element parameters) or simple shapes (which can be parameterized from EM > Component > Parameters).