I also tried a traditional url below, but did not work
Instead of making changes to core files, why not add a line to /cardinal-child/functions.php?
If not, then where would i change /cardinal/swift-framework/sf-megamenu/custom_walker.php?
(the svg icon does not have to go above the menu text, it can go to the right like ss-gizmo or font-awesome)
Or, how about just adding a simple button below the vertical menu?
li.menu-item-620.menu-item.menu-item-type-custom.menu-item-object-custom.sf-std-menu.sf-mega-menu-fw.sf-menu-item-loggedin.sf-menu-item-btn {
display: block;
text-indent: -9999px;
width: 100px;
height: 82px;
background-size: 100px 82px;
background: ‘https://s3.amazonaws.com/zocalo.pro/global/heavy/bucketfm.svg’;
}