How do I send key strokes to a window without having to activate it using Windows API?

Alright, this is kind of disappointing I'm sure, but you fundamentally cannot do this with 100% reliability.

Windows assumes that the active window is the one getting keyboard input. The proper way to fake keyboard input is with SendInput, and you'll notice that it sends messages to the active window only.

That being said, you can SendMessage WM_KEYUP, WM_CHAR, and WM_KEYDOWN messages and (depending on the WndProc receiving them) maybe get away with it. But remember, its going to break under some circumstances, period.

Sounds like you are using keybd_event() or SendInput(), which both send keystrokes to the currently active window. To direct keystrokes to a specific window, regardless of whether that widnow is focused or not, you need to find its HWND handle first, and then post appropriately-formatted WM_KEYUP/DOWN and WM_CHAR messages directly to it.

once you have the windows HWND, you can directly SendMessage() the WM_KEYDOWN and WM_KEYUP messages to its message queue. The window does not have to be active.

However, understand that this depends on how the target application processes keyboard input. There are several different ways to handle it.

WM_KEYUP/WM_KEYDOWN is most common and some applications only process one or the other (usually WM_KEYDOWN).

WM_CHAR is also fairly common

Some programs use GetAsyncKeyState, GetKeyState, or GetKeyboardState. This is extremely unusual, but effectively prevents keypress injection with SendMessage(). If this is the case fall back to keybd_event() which is directly handled by the keyboard driver. Of course the window will have to be active