add-cntent-on-custom-menu-page.php

<?php
function ld_dashboard_after_content_callback() {
	if ( isset( $_GET['tab'] ) && 'custom' === $_GET['tab'] ) { // We used the same slug here that we used while adding the menu i.e. 'custom'
		echo 'This is custom nav content';
	}
}

add_action( 'ld_dashboard_after_content', 'ld_dashboard_after_content_callback' );
?>