BCGControlBar Pro for MFC
UINT BCGM_GETWINDOWCORNERTYPE

Override BCGM_GETWINDOWCORNERTYPE message if you wish to modify the window corner type (Windows 11 or later).

Parameters
wParamThe default corner type (BCG_WINDOW_CORNER_TYPE).
lParamthis parameter is not used.
Returns
Corner type (BCG_WINDOW_CORNER_TYPE) or zero if you don't wish to modify the type.