Hello,
Open the icon you want to apply to the mapkey with an image editor (example Paint) select the fields you want from the icon (or select the entire icon) and Copy,
Open Proe and right click on the menu and Toolbars. From the Command toolbar find the mapkey you want (mapkey Categorie) click on the mapkey > right click > Paste button image.
If you adjust right your selected icon you will have good results
hope this helps
xcad