According to the API description you have to pass the hwnd of your taskbar. :/

SHAppBarMessage(ABM_QUERYPOS, pabd);

pabd
A pointer to an APPBARDATA structure. The uEdge member specifies a screen edge, and the rc member contains the proposed bounding rectangle. When the SHAppBarMessage function returns, rc contains the approved bounding rectangle. You must specify the cbSize, hWnd, uEdge, and rc members when sending this message; all other members are ignored.

Source: msdn