We have two choices for achieving this functionality.

One option is to directly add in topmenu.phtml by overriding a file in your theme. There are chances that it won’t work if you have a third party mega menu extension.

Another way is by using the plugin, Here we are going to talk about the same.

We can achieve our requirement by executing two steps:

First, you need to create di.xml in your custom module.

[Vendor Name]/[Module Name]/etc/di.xml

<!-- Add custom menu in navigation -->
<type name="MagentoThemeBlockHtmlTopmenu">
    <plugin name="add_registry_inmenu" type="[Vendor Name][Module Name]PluginCustommenu" sortOrder="10" disabled="false"/>
</type>

Now you have to create a plugin file in your custom module

[Vendor Name]/[Module Name]/Plugin/Custommenu.php

<?php

namespace [Vendor Name][Module Name]Plugin;

class Custommenu
{
    public function afterGetHtml(MagentoThemeBlockHtmlTopmenu $menu, $html)
    {
        $giftTopUrl = $menu->getUrl('giftr/search/result'); /* Here is the menu link which are redirect after click */
        $baseUrl = $menu->getUrl('*/*/*', ['_current' => true, '_use_rewrite' => true]);
        if (strpos($baseUrl,'giftr/search/result') !== false) {
            $html .= "<li class="level0 nav-5 active level-top parent ui-menu-item">";
        } else {
            $html .= "<li class="level0 nav-4 level-top parent ui-menu-item">";
        }
        $html .= "<a href="" . $giftTopUrl . "" class="level-top ui-corner-all"><span>" . __("Gift Registry") . "</span></a>";
        $html .= "</li>";
        return $html;
    }
}

After executing both steps, run below command:

php bin/magento cache:flush

Now you can check your navigation menu. Your custom menu is added after all top menus.