Assuming Button is a TButton * try this:
SendMessage(Button->Handle, WM_SETFOCUS, 0, 0);
SendMessage(Button->Handle, WM_KEYDOWN, VK_SPACE, 0);
SendMessage(Button->Handle, WM_KEYUP, VK_SPACE, 0);
PostMessage(Button->Handle, WM_SETFOCUS, 0, 0);
PostMessage(Button->Handle, WM_KEYDOWN, VK_SPACE...