Is there a way to get excel to add an event onto an outlook calender?

My spreadsheet is set up to have a few possible fields, but i want when 
'Annual Leave' is selected, an outlook calender all day event to be added.

Excel and outlook will be open at the same time so nothing needs opening.

I have the macro now to add the event onto my calender.

Sub Add_Appointment()
    Dim myOlapp As Object
    Dim myitem As Object
    
    Set myOlapp = CreateObject("Outlook.Application")
    Set myitem = myOlapp.createitem(1)
    
    With myitem
        .Body = "Annual Leave."
         '.Duration = dur'
        .AllDayEvent = True
        .Subject = "A/L"
        .Save
        
    End With
    
    Set myitem = Nothing
    Set myOlapp = Nothing
    
End Sub

What i now would like to know is:-

1) A code to run this macro if a field is changed, for example if a cell 
between range b1 to b14 changes to A/L i want this to run and add the 
appointment into my calender. This could also be done by clicking a button 
after any updates have been made.

2) What field do i need to change to select the calender i want this added 
to, i can get this to add to my calender, but eventually want it to add to a 
shared calender.

3) Is there a way to only add new appointments, and not repeating the old 
ones.

Thanks in advance