Unfortunately, at the moment there is no dedicated option/method, but I have a solution for you:

const modalAdd = calendarInstance._addEventModal;

modalAdd.addEventListener('shown.mdb.modal', () => {
  const modalBtnSave = modalAdd.querySelector('.modal .btn-add-event');
  const modalInputSummary = modalAdd.querySelector('.modal .calendar-summary-input');

  modalBtnSave.addEventListener('mousedown', () => {
    if (!modalInputSummary.value) {
      modalInputSummary.value = 'default value';
      modalInputSummary.dispatchEvent(new Event('input'));
    }
  }, {once : true})
})