To call it from C#, you need to use P/Invoke service to invoke the unmanaged function residing in user32.dll
private static extern bool RegisterHotKey(IntPtr hWnd, int id, int fsModifiers, int vk);
The hWnd parameter is the handle of a form that will receive WM_HOTKEY message. To get the handle of a Form simply use Handle property. In order to process WM_HOTKEY message, your form needs to override WndProc method.
Just one more thing you should know: RegisterHotKey and ShowInTaskBar property don't mix well. It seems that a new window is created each time ShowInTaskBar property is called so your WndProc mehod will stop recieving hot-keys. You need to RegisterHotKey again because Handle property returns a new window handle after ShowInTaskBar was called.