WM_LBUTTONDOWN is for left mouse clicks and WM_CHAR is for characters, not keypresses. So you need WM_KEYDOWN. But you don't put anything in it; it's a constant that indentifies the message. Something like this:
invoke PostMessage, [hWnd], WM_KEYDOWN, VK_SPACE, 0
invoke Sleep, 50
invoke PostMessage, [hWnd], WM_KEYUP, VK_SPACE, 0xC0000001