Howto add a custom field in the closure notes ?
What are you referring to as a "closure note"?osTicket does not have a closure note, or closure notification.
i want to add a field as "Spare Replacement yes or no" Field while closing the ticket
So do you want to add this as a status or as an extra field below the status / post reply text box?
i want to add extra field below the status box
That is not a feature in the osTicket ui at this time.