How to disable the full screen of another program?

For example, a game that is full screen (with full screen does not mean maximized, but "fullscreen same"). How do I get into windowed mode with C ++?

I've tried using "SendMessage":

#include <windows.h>
int main()
{
    while (true)
    {
        if (GetAsyncKeyState(88))
        {
            HWND hWnd = GetActiveWindow();
            SendMessageA(hWnd, WM_SYSCOMMAND, SC_RESTORE, 0);
        }
    }
}

But it did not work.