add_action('admin_menu', 'custom_admin_menu');
function custom_admin_menu() {
//internal relative url
add_menu_page( 'inschrijvingen_link', 'Inschrijvingen', 'manage_options', 'e-form-submissions', '', 'dashicons-email', 2 );
//external url
add_menu_page('google_link', 'Google', 'read', 'https://google.com/', '', 'dashicons-text', 1);
}