Capture non-client mouse click on sizable window

[C++ Programming]

I want to have the user click on a MDIchild window frame to resize it, but when you set the frame to WS_THICKFRAME, to get the resizable frame, then the OnNcLButtonDown() event is not seen. I just want to capture the mouse up so I know when the resize should stop. There must be a way to do this.