You would add the icon with a custom class like login
then edit the CSS to this:
/* Primary navigation styles */
.main-navigation:not(.slideout-navigation) .main-nav {
flex-grow: 1;
}
.main-navigation .main-nav {
text-align: center;
}
@media (min-width: 769px) {
.nav-aligned-right .main-navigation .menu li.search-item,
.nav-aligned-right .main-navigation .menu li.wc-menu-item,
.nav-aligned-right .main-navigation .menu li.login {
float: right !important;
}
}