How To Add Favicon icon to the WordPress Admin & Login Pages
Add following in your theme’s functions.php file, or into a custom plugin:
function add_favicon() {
$favicon_url = get_stylesheet_directory_uri() . '/images/favicon.ico';
echo '<link rel="shortcut icon" href="' . $favicon_url . '" />';
}
add_action('login_head', 'add_favicon');
add_action('admin_head', 'add_favicon');