This help doc will demo how to sell comp tickets in the box office and add more comp tickets types if your organization requires them.
In the box office, select the number of seats or the seats you would like from a seat map you'd like and a menu below will appear. Under ticket name, you can open up a drop-down menu that looks like this:
From this menu, you can select the ticket to be a "Comp Ticket" this will set the price to zero and waive all fees.
Custom comp tickets can also be set up using box office ticket types. How to set up and use special box office only ticket types
If your event is free - simply set the price of the item to zero during set up.
Video Demo