Add a custom logout link to the main menu for logged in users

functions.php

<?php
function wti_loginout_menu_link( $items, $args ) {
   if ($args->theme_location == 'primary') { // Adjust this to the menu location.
      if (is_user_logged_in()) {
         $items .= '<li class="right"><a href="'. wp_logout_url() .'">'. __("Log Out", "textdomain") .'</a></li>';
      }
   }
   return $items;
}

add_filter( 'wp_nav_menu_items', 'wti_loginout_menu_link', 10, 2 );