C++ Send Keys Background

As Entropia said, Use SendMessage. You can do it followed by WM_LBUTTON.

Code:

LRESULT WINAPI SendMessage(
  _In_  HWND hWnd,
  _In_  UINT Msg,
  _In_  WPARAM wParam,
  _In_  LPARAM lParam
);

You'll have something like this then:

Code:

#include <Windows.h>
HWND window = FindWindow(0, L"Your Window Title");
SendMessage(window, WM_LBUTTONDOWN, MK_LBUTTON, MAKELPARAM(x, y));
SendMessage(window, WM_LBUTTONUP, MK_LBUTTON, MAKELPARAM(x, y));

EDIT; I'm sorry I thought you wanted to send a mouseclick.
Well, here this is how you send the "x" key:

Code:

HWND window = FindWindow(0, L"Your Window Title");
SendMessage(window, WM_CHAR, 'x', 0);

Also, if you want to send a keystroke with SendMessage you can do something like this:

Code:

void sendText(LPCWSTR windowName, char* szText)
{
	if (strlen(szText) < 1) return;

	for (int i = 0; i < strlen(szText); ++i)
	{
		HWND hwnd = FindWindow(0, windowName);
		SendMessage(hwnd, WM_CHAR, szText[i], 0);
	}
}

Code:

sendText(L"Your Window Name", "some text here");