Hi Yuan, this should do it in the current MoI v4 beta:

				<moi:CommandMenuButton menu="FileMenu.htm" icon="icons/FileIcon.png"
					onmouseenter="
						var e = document.createEvent( 'MouseEvent' );
						e.initMouseEvent( 'mousedown', false, false, window, 0, window.event.screenX, window.event.screenY, window.event.clientX, window.event.clientY, false, false, false, false, 1, null );
						this.dispatchEvent( e );
					"><moi:Text textid="File"/></moi:CommandMenuButton>

For V3 use onmouseover="" to trigger it instead of onmouseenter=""