Does anyone know how to send only "Alt" key signal by XKBD, not a letter with Alt like "Alt + x"?

Thanks to the great folks here, I can run HP200LX simulator on N810 via DOSBOX. . One problem is that the simulator uses "Alt" key to activate 'Menu' but XKBD seem not send "Alt" key itself but rather wait for another key and send as "Alt+x".