If you want to allow constituents to use the additional donation field in an event, with out purchasing a ticket follow these steps.
  1. Choose to edit the event.
  2. Create a new ticket type with a $0 value.
  3. Add some information on the ticket page, that explains for an additional donation to be made, you will first have to purchase 1, $0 donation ticket.
This way when constituents visit the event, if they don't want to attend, but still want to donate. They can choose to buy the $0 donation ticket you created, and make an additional donation with it.