Reply To: Top bar

You could add it as a menu item in the header menu, and then use custom CSS (Appearance > Customize > Additional CSS) to make it appear as a button:

#primary-menu > li:nth-of-type(3) {
    background: red;
    border-radius: 3px;
}

In the example above, change the background color to any color you want, and change the number in brackets (3) to the number of the menu item. For example if it is the 4th item use number (4) and so on.