Per MSDN SendMessageCallback
Func __WinAPI_SendMessageCallback($hWnd, $iMsg, $wParam, $lParam, $pCallback, $pData = NULL)
Local $aResult = DllCall("user32.dll", "int", "SendMessageCallback", _
"hwnd", $hWnd, _
"uint", $iMsg, _
"wparam", $wParam, _
"lparam", $lParam, _
"ptr", $pCallback, _
"ulong_ptr", $pData)
If @error Then Return SetError(@error, @extended, 0)
Return $aResult[0]
EndFunc
Note: I haven't decided if its better to send $pdata as a dword or as a uint_ptr or just exclude it all together