custom-nav-my-dashboard.php

<?php
function ld_dashboard_add_nav_menu( $menu_items ) {
	$menu_items['all']['custom'] = array(
		'url'   => get_the_permalink() . '?tab=custom', // tab=custom sets the slug of your custom tab which will later be used to display content in this tab.
		'icon'  => '<img src="http://lddashboard.local/wp-content/plugins/ld-dashboard/public/img/icons/tachometer-alt.svg">',
		'label' => 'Custom Nav',
	);
	return $menu_items;
}
add_filter( 'learndash_dashboard_nav_menu', 'ld_dashboard_add_nav_menu', 10, 1 );

?>