To add a button on the menu of the Divi theme, find the menu item ID by right clicking and 'inspect element.' Then add the below code to custom css in theme options.

li#menu-item-61 a {
padding: 0.3em 1em;
border: 2px solid;
margin-bottom: 20px;
-webkit-border-radius: 3px;
-moz-border-radius: 3px;
border-radius: 3px;
background: #407CD4;
color: #ffffff;
-webkit-transition: all 0.2s;
-moz-transition: all 0.2s;
transition: all 0.2s;
height: 25px;
padding-bottom: 20px !important;
}