Getting Pointer to the "Desktop" Window, to use XSelectInput () API

Dear all,

I need to use XSelectInput() API of Xlib.

The window which i am interested is the desktop and the event is when
the Left button of the mouse is clicked. (in short, when a user left
clicks on the desktop).

The Second argument to the XSelectInput is Window i.e
   int XSelectInput(Display *display, Window w, long event_mask);

I would like to know, is it Ok
to use display = XopenDisplay(NULL); for the first argument ?
&
how do we get the second argument of the API XSelectInput() i.e Window w ?

Your replies would be very much appreciated.

Regards,
Vishal.