Change message class to appointments to custom form

Question

When I receive a new appointment request I want to add a form for me to complete (client name, sales opportunity, territory) I created a form in Developer and it works on appointments I create but does not appear on appointments I receive. I tried the following with no joy. My form is NewForm:

 Sub cItems_ItemChange(ByVal Item As Object)
  [enter image description here][1]
   ' Change the following line to your new Message Class
  NewMC = "IPM.Appointment.NewForm"

  If Item.Class = olAppointment Then
       If Item.MessageClass <> NewMC Then
          Item.MessageClass = NewMC
          Item.Save
  End If
  
 End Sub

This works on appointments I create: https://i.stack.imgur.com/B4pUL.png