From Xojo Documentation
Appends Item in a new row to the end of the list.
You can add a "-" on macOS (instead of calling PopupMenu.AddSeparator) to add a separator. Windows and Linux do not support separators within PopupMenus, so this simply adds a row with the text "-" that can be selectable by the user.
The following example in the Open event of the control adds two rows to the menu.