WP Add Admin Page

functions.php

/*
 * Add Admin Hidden Page.
 */
add_action( 'admin_menu', function () {
	add_submenu_page(
		'',
		__( 'Trade License Image', 'text-domain' ),
		__( 'Trade License Image', 'text-domain' ),
		'manage_options',
		'trade-license-image',
		function () {
			isset( $_GET['image'] ) or wp_die( '<div class="wrap"><h1>Nothing to Show on this page</h1></div>' );
			
			$image      = sanitize_file_name( $_GET['image'] );
			?>
			<div class="wrap">
				<h1><?php _e( 'Welcome to my custom admin page.', 'text-domain' ); ?></h1>
			</div>
			<?php
		}
	);
} );

hooks.php

/**
Dashboard: ‘index.php’
Posts: ‘edit.php’
Media: ‘upload.php’
Pages: ‘edit.php?post_type=page’
Comments: ‘edit-comments.php’
Custom Post Types: ‘edit.php?post_type=your_post_type’
Appearance: ‘themes.php’
Plugins: ‘plugins.php’
Users: ‘users.php’
Tools: ‘tools.php’
Settings: ‘options-general.php’
Network Settings: ‘settings.php’
 */
add_action( 'admin_menu', function () {
	add_submenu_page(
		'users.php',
		__( 'Trade License Image', 'text-domain' ),
		__( 'Trade License Image', 'text-domain' ),
		'manage_options',
		'trade-license-image',
		function () {
			isset( $_GET['image'] ) or wp_die( '<div class="wrap"><h1>Nothing to Show on this page</h1></div>' );
			
			$image      = sanitize_file_name( $_GET['image'] );
			?>
			<div class="wrap">
				<h1><?php _e( 'Welcome to my custom admin page.', 'text-domain' ); ?></h1>
			</div>
			<?php
		}
	);
} );