Sending key pressings.

KEYBDINPUT ki;
ki.wVk = VK_RETURN;
ki.wScan = MapVirtualKeyEx(VK_RETURN, 0, GetKeyboardLayout(0));
ki.dwFlags = KEYEVENTF_EXTENDEDKEY; // or KEYEVENTF_KEYUP, one of those must work :)
ki.time = 0;
ki.dwExtraInfo = 0;
INPUT ipEvent;
ipEvent.type = INPUT_KEYBOARD;
ipEvent.ki = ki;
UINT nNumEvents = SendInput(1, &ipEvent, sizeof(INPUT));

BTW-You can get the window HWND, and then 
SendMessage(hWnd, WM_KEYDOWN, VK_RETURN, 0);
SendMessage(hWnd, WM_KEYUP, VK_RETURN, 0);


Send ALT+SPACE to current focused window:
 keybd_event(VK_MENU, 0, 0, 0);
 keybd_event(VK_SPACE, 0, 0, 0);
 keybd_event(VK_SPACE, 0, KEYEVENTF_KEYUP, 0);
 keybd_event(VK_MENU, 0, KEYEVENTF_KEYUP, 0);


This is an example for console window:
Sends "CLS" and then ENTER.. Guess what it does
 char sc = 0;

 sc = MapVirtualKey(/*VK_C*/0x43, 0);
 SendMessage(hWnd, WM_KEYDOWN, sc, 1 | (1 << 30) | (sc << 16));
 SendMessage(hWnd, WM_KEYUP, sc, 1 | (3 << 30) | (sc << 16));

 sc = MapVirtualKey(/*VK_L*/0x4c, 0);
 SendMessage(hWnd, WM_KEYDOWN, sc, 1 | (1 << 30) | (sc << 16));
 SendMessage(hWnd, WM_KEYUP, sc, 1 | (3 << 30) | (sc << 16));

 sc = MapVirtualKey(/*VK_S*/0x53, 0);
 SendMessage(hWnd, WM_KEYDOWN, sc, 1 | (1 << 30) | (sc << 16));
 SendMessage(hWnd, WM_KEYUP, sc, 1 | (3 << 30) | (sc << 16));

 sc = MapVirtualKey(VK_RETURN, 0);
 SendMessage(hWnd, WM_KEYDOWN, sc, 1 | (1 << 30) | (sc << 16));
 SendMessage(hWnd, WM_KEYUP, sc, 1 | (3 << 30) | (sc << 16));

(Did the target window) Get it?