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