It seems that you will have to send the notification message manually. This is what MSDN has to say about trackbar notification messages:
Originally Posted by
MSDN Trackbar Notification Messages
A trackbar notifies its parent window of user actions by sending the parent a WM_HSCROLL or WM_VSCROLL message. A trackbar with the TBS_HORZ style sends WM_HSCROLL messages. A trackbar with the TBS_VERT style sends WM_VSCROLL messages. The low-order word of the wParam parameter of WM_HSCROLL or WM_VSCROLL contains the notification code. For the TB_THUMBPOSITION and TB_THUMBTRACK notifications, the high-order word of the wParam parameter specifies the position of the slider. For all other notifications, the high-order word is zero; send the TBM_GETPOS message to determine the slider position. The lParam parameter is the handle to the trackbar.
Reading this "from the other side" we should be able to manually construct and send the three notification messages that the target application is most likely to be responding to:
Code:
apptr1->SendMessage(WM_HSCROLL, (WPARAM) MAKELONG(TB_THUMBTRACK, 100), (LPARAM) (HWND) apptr1->GetDlgItem(0x42B));
apptr1->SendMessage(WM_HSCROLL, (WPARAM) MAKELONG(TB_THUMBPOSITION, 100), (LPARAM) (HWND) apptr1->GetDlgItem(0x42B));
apptr1->SendMessage(WM_HSCROLL, (WPARAM) MAKELONG(TB_ENDTRACK, 0), (LPARAM) (HWND) apptr1->GetDlgItem(0x42B));
It should be noted that notification messages are sent to the parent window. As a style issue, you should consider defining constants for control ids rather than using magic numbers.