How do I send a WM_GETDLGCODE message?

So,I have  a gdi drawn list,and when i press the up and down arrows,these messages are sent to it:

WM_GETDLGCODE

WM_GETDLGCODE fuDlgCode:DLGC_WANTARROWS

I already checked the msdn page for the message: http://msdn.microsoft.com/en-us/library/windows/desktop/ms645425(v=vs.85).aspx

And I'm sending this message to the list(the list is a child):

PostMessage(hwnd,WM_GETDLGCODE,VK_UP,NULL);

It should select the item above,but obviously,that doesn't happen,what can I do?