Is it possible to put a custom field in the pop-up status window on ticket closing? So, we create a custom field that must be filled before closing the ticket, and want to put it in the pop-up window which come up during the ticket closing process.
It's not possible to have something pop-up when closing a ticket, however you can mark a form field as required entry (called data integrity) before the ticket can be closed.
See here: https://forum.osticket.com/d/98705-add-category-while-closing-an-osticket