Can I send a user-defined message to a window's Dialogproc function???

I want to send a WM message to a dialogproc of another dialogbox. To explain in detail, When I press mouse button within the window area of one window it should send a user-created WM message coz the other dialog box needs to be updated. Can someone explain to me how can this be done??

Is WM_APP useful for this??? If yes can someone give links to good resource to read more about it?

Thanks a lot.

http://www.codeproject.com/Articles/...age-Management
http://msdn.microsoft.com/en-us/libr...(v=vs.85).aspx