Subscribe Button

Not to disagree with Doug, but I would suggest a differenty way.  First, you can add to the menu but not the menu bar. Adding subscribe to the menu is unnecessary as it is already there. The menu bar is not editable by any means.  Second, here is how I would acheive what you are looking for.

1) Add your new component directly above the menbar component. In this example, my custom component is called "custom-add-to-my-places"

2) Add css to float the custom component and the menu bar to the left. You may need to adjust the width of menu bar:

.lia-component-custom-add-to-my-places {
    float: left;
}
.lia-menu-bar-top {
    float: left;
    width: 97%;
}

3) You  now have the look of the two items living in the same line. In my case, I styled the subscription as a "favorites" heart to acheive an all icon look: