When a selected IME needs information about the composition window, the IMR_COMPOSITIONFORM message is sent to the application through the WM_IME_REQUEST message.
lParam =-(LPARAM) lpCOMPOSITIONFORM;
Returns a nonzero value if the application filled in the COMPOSITIONFORM structure in lParam. Otherwise, returns zero.
This notification may be sent by the IME to a window that cleared the ISC_SHOWUICOMPOSITIONWINDOW flag in the WM_SETCONTEXT message handler.
Input Method Editor Overview, Input Method Editor Messages, WM_IME_REQUEST