1. Add a regular menu item via to your menu via Appearance>Menus section.
2. Add the class "button" to the "CSS Classes" field.

3. Style the button with the next CSS codes by adding them to Theme Options>Asvanced options>CSS code field:
/*Added buttons in Main Menu*/#mainMenu > ul > li.button { font-size: initial; height: auto; line-height: initial; margin: initial; border: none; background-color: transparent; padding: 0 15px; position: relative; margin-left: 2px;}#mainMenu > ul > li.button > a { position: unset;}#mainMenu > ul > li.button > a:before { content: ""; display: block; position: absolute; width: 100%; height: 25px; left: 0; top: 50%; transform: translateY(-50%); border-radius: 3px; z-index: -1; background-color: #CA0027;}@media (max-width: 991px) { #mainMenu > ul > li.button { padding: 0; margin-left: 0; } #mainMenu > ul > li.button > a { display: inline-block; padding: 7px 20px; margin: 10px 0; background-color: #CA0027 !important; color: #fff !important; height: auto; line-height: 1; border-radius: 3px; } #mainMenu > ul > li.button > a::before { display: none; }}4. Change CSS values (like background-color etc) according to your needs.