To highlight a specific menu item you need to enable “CSS classes” feature under the theme options on “menus” page. To do that simply goto Appearance->Menus from your dashboard and click on “Screen Options” on top right side of your screen and check “CSS classes” field.
After you enable the feature, goto the menu item you want to add the button and under “CSS Classes (optional)” field enter “menu-button”.
After that click save and refresh your website.