The context menu usually sits on your main window. To get it, just call mainWindow.ContextMenuand then you get a Menu element back with which you can interact.
By default, it uses the algorithm of the UI-framework (WPF, WinForms, Win32, ...) of the window. In certain cases (let's say you have a Win32 application (with a Win32 main window) and embedded a WPF control which provides the context menu) you need to tell which framework algorithm you want to use. For this, the Window provides the method GetContextMenuByFrameworkType where you can pass your desired framework type to get the context menu you need.