add_action('admin_menu', 'hrb_calendar_admin_menu');

function hrb_calendar_admin_menu() {
    add_menu_page(
        'Agenda HRB',
        'Agenda HRB',
        'manage_options',
        'hrb-agenda',
        'hrb_agenda_admin_page',
        'dashicons-calendar-alt'
    );

    add_submenu_page(
        'hrb-agenda',
        'Nieuw evenement',
        'Nieuw evenement',
        'manage_options',
        'hrb-agenda-add',
        'hrb_agenda_add_page'
    );
}