WM_NCRBUTTONDOWN

This message is sent to a window when the user presses the right mouse button while the cursor is within a nonclient area of the window.

Parameter Description  

wParam Contains the code returned by WM_NCHITTEST (for more information, see the WM_NCHITTEST message, earlier in this chapter).  
lParam Contains a POINT data structure that contains the x- and y-screen coordinates of the cursor position. These coordinates are always relative to the upper-left corner of the screen.