Code.gs

function rentalsToCalendar() {
    
  var spreadsheet = SpreadsheetApp.getActiveSheet();
  var calendarId = spreadsheet.getRange("C4").getValue();  
    
  var eventCal = CalendarApp.getCalendarById(calendarId);

  var rentalList = spreadsheet.getRange("A8:C12").getValues();
  
  for (x=0; x<rentalList.length; x++) {
    
    var rental = rentalList[x];
    
    var startTime = rental[0];
    var endTime = rental[1];
    var eventName = rental[2];
    
    eventCal.createEvent(eventName, startTime, endTime);

  }

}



function onOpen() {
  var ui = SpreadsheetApp.getUi();
  ui.createMenu('Sync to Calendar')
      .addItem('Add rentals now', 'rentalsToCalendar')
      .addToUi();
}