I'm going to be cautious about answering the questions directly because I think you may not have the terminology correct.
When you open a Patran application area like "Geometry" it creates a "Form" that is displayed and this has "widgets" on it of various types. Almost always are some buttons like the "APPLY" button.
I may be wrong but I do not think you were asking how to assign the PCL function to this type of button - to do that it is just the "call back function" that you specify when you create the button.
I suspect that what you want to do is either to make a new menu item on the top menu bar or to create a new icon on a tool bar.
There are a couple of articles to get you started, on user defined menu items and toolbar icons.
The PCL manual sections do have lots of details but these articles give you a good introduction to specific things.
