As this may help you and other people to bypass the PostMessage() blocking, the "smart" devs forgot about the virtual keyboard which is available on Windows "
osk.exe".
You just need to run osk.exe, put the game window on foreground
Code:
SetForegroundWindow(hwnd);
and click on the virtual keyboard.
Clicking on the virt. keyboard could be done manually using the mouse move and the left-click but this will require to know the position of every button on the virt. keyboard window, but I am pretty sure it can be reversed and you will be able to call the virt. keyboard buttons from your c++ code